Are there any good basic killer builds not behind pay walls?

First question would be "are they on PC or console?" Console gets the Doctor, whereas PC doesn't. Wraith is probably the best to start with, but Huntress and Trapper would be recommended to get a feel for different classes.

If they have the Doctor, Overcharge is often overlooked and certainly a good basic gen regression perk.

If they've leveled them a bit and can mix and match I'd probably try something like;

Nurse's Calling (Nurse) - the only aura perk I think they'll have, but a very good one.

Lightborn (Hillbilly) - Not essential, but helpful if you're starting on killer

Brutal Strength (Trapper) - Breaking stuff faster (gens and pallets) can help with chases

After that, they're a bit limited. I wouldn't recommend Hex's early on, but they do have Hex : Ruin and Hex : Devour Hope on Hag which can be powerful. If they're playing someone other than Wraith, I'd probably use Tinkerer (Hillbilly) as the undetectable proc will be very useful if they can get used to it.

The most solid excluding DLC perks is Wraith with anything from: Sloppy, Nurse's, Enduring, Brutal, Jolt, Fearmonger.

Agitation, Brutal Strength, Sloppy Butcher or either NOED or JOLT

Pretty decent build on most m1 killers

I'm going to address singular perks first by killer first, because boy, do I love compartmentalizing my information dumps!

From the Trapper, Agitation and Brutal Strength I feel are his better perks, though obviously "better" is completely subjective and combined in certain ways, weirder perks can work pretty well together. Even if you're not trying to make it to a Scourge Hook, Agitation can reduce time spent dragging someone to a hook, so while hardly the most powerful of perks, it can be reliably and consistently utilized by pretty much every killer. And of course, most killers are going to be breaking pallets at some point, so why not be faster doing so?

From the Wraith, Bloodhound and Shadowborn are guilty pleasures of mine. Once a survivor is injured, it's hard to lose track of them running Bloodhound, and it's always fun to see neon red paint from a good distance away to smack some injured survivor who thought they were hidden. Shadowborn is its own debate about whether it's good or not, but it's certainly fun for me to slap on, personally, though it's very much a personal preference thing.

From the Hillbilly, all of his perks are pretty good. Enduring is great for people who constantly get slapped by pallets, Lightborn can protect you from insane flashlight saves that you weren't expecting, and Tinkerer can be deadly on certain killers under the right circumstances.

From the Nurse, Stridor can be fun, but a lot of it depends on your playstyle. I personally don't see much of a point to Thanatophobia, but again, personal preference. A Nurse's Calling is incredibly useful. I haven't seen a lot of people run it in a while, but it's always been a perk that I'll slap on if not sure what my fourth perk slot should be.

From the Huntress, Territorial Imperative has the potential of giving you an early down or doing basically nothing all game. It's always fun seeing someone go into the basement to loot the chest and give them some rapid succession smacks, an instant down with certain killers, or trap them in there for another easy down with killers like the Trapper. And basement builds can make use of it, too. For most other circumstances, I personally don't get much use out of it, but maybe that's just me. Beast of Prey is another guilty pleasure of mine. Going Undetectable mid-chase can really confuse survivors, even if it's only for a split second. But sometimes a split second is all you need. Hex: Huntress Lullaby is... a hex. It could do you lots of favors, or it could get cleansed in the first twenty seconds of the game. Personal preference and sheer dumb luck with location spawns is a pretty big factor there.

Now on to the common killer perks, many of which I think are severely underrated, so this'll be fun!

Bitter Murmur isn't a perk I run often, but it can be pretty sweet even for killers who aren't the Huntress. It can be good to know which direction survivors are going, if they're maybe healing an injured teammate right there, or if they're hanging around the generator for some similar reason like cleansing or blessing a totem.

Claustrophobia is a perk... once in a situational blue moon.

Deerstalker can be nice for keeping track of people on the ground, though A Nurse's Calling can do basically the same thing if that person's being healed at any given moment and has other utilizations.

Distressing can be fun in combination with other perks, and who's going to say no to more bloodpoints?

Fearmonger isn't one I use too often, though I feel like a lot of people only run it for the exhaustion portion of it and forget the blindness aspect of it. Blindness probably isn't as potent as it used to be considering that as a survivor, you always know what action your teammate is performing in that instant. Still, keeping survivors from having information in any way can be devastating under the right circumstances.

Hex: No One Escapes Death is one I'm pretty sure I've never even run before. Not because of the view a lot of people seem to have that if you're running this perk, you're automatically morally reprehensible or just a bad killer. I'd just rather have a perk that'll help me in some way throughout the trial than give me an edge once the generators are done. But again, personal preference.

Hex: Thrill of the Hunt can be nasty, especially paired with Hex: Undying, though I understand your friend doesn't have access to this perk. Something I do sometimes is run Hex: Thrill of the Hunt on its own. If survivors want to slap a boon on a totem, they're going to have to dedicate more time to it. And sometimes running Thrill of the Hunt gives survivors the illusion that you're trying to protect a more lethal hex, such as Devour Hope. It can be entertaining when survivors seem to be spending more time booning totems or looking for this mysterious other hex totem when, in fact, there isn't one. And just keeps people off of generators in general for a time, which is always valuable to the killer.

Insidious has the reputation for being used by Leatherface or similar killers camping the basement hook. I've seen people utilize it in other ways that I think are pretty clever, though, so it's a perk I'd like to devote some time to finding its weird, situational uses. I wouldn't really recommend it for someone either learning the killer or with few perks to work with in general, though.

Iron Grasp is another guilty pleasure of mine. Agitation and Iron Grasp is a commonly known combination, and Iron Grasp by itself is enough to completely destroy Boil Over. I regularly end up using Iron Grasp when I just don't really have anything else I want to use in a current build.

Jolt can obviously be used for its regression, though sometimes it's nice to just know when there are nearby generators that have progress without having to waddle up to them. So pretty solid, I think, regardless of whether you use it for regression or information.

Scourge Hook: Monstrous Shrine can be nasty even on its own. Survivors can overestimate how much time they have to unhook a survivor even without it, and Monstrous Shrine just makes this even more likely. I personally don't like running this perk because I generally want someone to die from having chased and hooked them three times, not due to their team's negligence. More points and more fun for me and everyone involved, but that's just my outlook.

Shattered Hope can be alright even if you don't particularly mind boons or pair it with Hex: Pentimento. For survivors who aren't extremely acquainted with different maps' totem spawns, this can be a huge time sink for them when they realize the totem they managed to find is no longer an option. It won't do much against survivors very familiar with totem spawns or on maps where they're just in more predictable or visible places, but the occasional aura reading can be helpful, too. And if the boon is one of the other three aside from Circle of Healing, it can force Shadow Step, Dark Theory, and/or Exponential into a less strategic position for survivors. But Circle of Healing is definitely the boon you'll be encountering the most.

Sloppy Butcher is one of those perks that I feel like just works on almost every killer. Making healing more of a time-sink is obviously less time survivors can spend on generators, and it can work in your favor as the killer in other ways, too. It's been a while since I've run it, but I'm pretty sure if you snuff a totem while someone's healing themselves with Circle of Healing, all of their progress is almost instantly lost and it acts as what basically amounts to a missed skill check for them, too, revealing their location to you. So pretty solid, in my mind.

Spies From The Shadows is another guilty pleasure. Catching survivors off guard can make all the difference, and unless they're running Calm Spirit, there's no really reliable way to avoid this without avoiding certain parts of the map. So pretty good, if a bit situational.

Unrelenting is really good for people who swing and miss a lot, myself being among them. But another neat thing you can do is kind of bait a dead hard. Tapping the attack button for a quick swing can make survivors panic and use their dead hard, and since you can recover faster, you can do this in a lot of areas without risking them making it to a safer area anyways.

Whispers! All hail the mighty Entity yelling crap in our heads! It can be annoying in some circumstances, like walking away from someone you just hooked and not being able to tell if the Entity's screeching at you about another survivor or the one you're actively walking away from. But in a lot of other instances, it can be a good way of saving time walking to generators (because if the Entity's not hollering at you, then there's no survivor nearby, so no need to walk all the way to that corner generator if you're close enough already.) And for killers who can control whether they're undetectable or not, like the Pig, it can tell you when to start crouching so you're not crawling around on the ground the entire match while generators are popping all around you.

Now for the builds!

I already mentioned Agitation and Iron Grasp. Useful on almost any killer, really, and makes getting to a Scourge Hook all the easier. So your friend could slap on Scourge Hook: Monstrous Shrine and see how that works, too. It might trick saboteurs, too. Some saboteurs learn where the Scourge Hooks are and plot their sabotage accordingly. If it doesn't really have much of an effect beyond faster Entity progression, however, you could easily just walk over to another hook, wasting a potential saboteur's time. And with Agitation and Iron Grasp, that's more time you have before a carried survivor escapes, so you could even go for a Scourge Hook knowing a saboteur's nearby. It could end in a free smack or down depending on how insistent that guy is on getting that hook sabotaged. You could also slap on Territorial Imperative if you want to do a basement build.

Sloppy Butcher and A Nurse's Calling can synergize well. If you see someone healing, not only can you interrupt them, but also ensure that they've lost all of their healing progress. So even if the injured survivor gets away, they'll still have to get that lost healing progression back.

Very situational, but Tinkerer and Claustrophobia can work sometimes. Either you get to the generator in progress and interrupt it before it's finished, or you get there too late and bar everyone in the immediate vicinity from windows. No one really uses Claustrophobia, so I imagine you could catch a lot of people off guard this way. It's still very situational, though, so that's up to your friend if they want to experiment with that or not.

A combination of Brutal Strength and Enduring has always been a favorite of mine. Barely batting an eye at pallet stuns and chipper-shredding through pallets may not be subtle, but it can certainly make you feel like an unstoppable force.

I could go on, but in the end, a lot of it has to do with your friend's playstyle as the killer. Are they more aggressive in chasing survivors, or defensive in prioritizing defending generators? I feel like I could give more relevant advice knowing what their strategy is, if they feel like sharing. :)

I am not here to discuss what a paywall is and what not but you can grind for it so i thought it's maybe nice to know, because the real paywall for my understanding is buying with auric cells. However, hope that brought you further, good luck.

Sloopy butcher + Nurses calling + Jolt + Fearmonger its a good start.(I usually use this with some killer like Dredge)

Some slow by butcher on survs, some regresion from jolt and some info for nurses.

Add brutal or agitation or some perk your friend want to try instead of fearmonger to add variety.

Of course your friend should at least give 1 lvl to each killer, but thats not a lot these days.

Spirit fury, enduring, bamboozle and pain resonance on m1 killers.

I use this on wraith, which I main and consistently get good results. Alas, addon dependant.

I also use this on doctor and works very well.