I have a player who is really stumped for the archetype to take at 3rd. He's playing the variant Feral Tiefling from Swordcoast and is hell-bent (pun intended) on vengeance against those who wiped out his young gang/guild.

Looking for a good fit and maybe something that builds onto the infernal aspect rather than just a general rogue sub.

Thoughts?

Before I give my answer, I have to ask which variant they chose.

Arcane tricker would be the obvious choice I think, in most cases. Adding spellcasting to the class would be thematically appropriate for any teifling. If they took on arcane initiate (warlock) as a feat at level 5 then that would cement it further. 

I was thinking Arcane Trickster, but if they took the Winged Option then I think I would go with Inquisitive. There just seems to be something “devilish” in being able to more easily discern lies, and read people so well that you can  Sneak Attack even without Advantage.

And depending on on what kinda tactics they prefer (ranged or melee) I would recommend different feats. For a melee build, definitely the Mobile feat for flybys, and it will eventually dovetail nicely with Steady Eye. For a Ranged Build, obviously other choices might be better.

Devil's Tongue (the one with Vicious Mockery, Charm Person, etc.)

Quote from JimFlory >>

Devil's Tongue (the one with Vicious Mockery, Charm Person, etc.)

 Since those spells use Charisma for Devil’s Tongue, I am assuming that your player’s Cha is relatively decent. Perhaps a Swashbuckler? That might play nicely with a Charismatic Tiefling. Especially if they take a small dip into o Warlock for Hexblade as those powers will not only complement the build, but also the devilish theme. Of course a small dip into Bard for either Whispers or Blades would meet the same criteria.

Of course, if you are open to UA, either of those two subclasses might be more what you are looking for.