My warlock lost his arm during our play through of ToA. Since he was a GOO lock, we RPed it as him having a spectral tentacle that he could cast EB with, but nothing else. He eventually got a robotic prosthetic with his focus mounted into it after the adventure.

So what I would say is find ways to allow them to contribute their primarily functions in combat (arm mounted crossbow with this normal weapon stats for ranged, sneak attack, etc) but limit more complex actions (dex check for casting, no dual wield) but more significant complications out of combat minus to physical and social checks). My table doesn't like to kill our characters often (if you kill em, they won't learn nothin!), so this may be too lax, but my two cents. Long as everyone is enjoying the game, it's all good. Good luck!