I just got Lily as a companion. I like to sneak and snipe and am playing on hardcoe, but I heard her companion quest required her to take a lot of damage. As that's not likely to happen with my playstyle (unless we both die), I decided to carefully get her into a situation were it would trigger, then I could quickly eliminate the enemy and keep her from dying.
So, I send her at some cazadors and lie in wait. All goes as planned until afterwords when I notice that her HP is suddenly extremely low. I open up her companion wheel and use a stimpack, but instead of healing her, her health drops to -650+ and she drops dead. Crazy, right? So, I reload and try again. This time I constantly check her HP after the fight and find it's constantly dropping. She's been poisoned, something I never had a problem with before. It shows no signs of stopping so I try a stimpack again. Doesn't work. Dead Lily. I try yet again, this time putting an antivenom in her inventory but she doesn't use it. Even from full health, the venom takes her all the way down to zero and she dies.
Granted, this is avoidable: save before fighting cazadors (the cliff racers of New Vegas, I tell ya) and radscorpions, don't let your companions get hit. That said, a more fluid, less frustrating fix would really be a godsend. Any ideas?
EDIT: Just like to note, I've never had this problem before. Maybe a recent patch did it, as it's also been a while since I fought cazadors (thank god).