I had an old boss who would say to me ...
If you associate with people above you, it will be uncomfortable, but they'll bring you up.
If you associate with people "below" you (in values, ethics, discipline, whatever), they will drag you down.
I've told my niece this so many times. I think making that decision to strive against one's baser instincts is hard - easier if you are struggling upwards with others - but almost impossible if you also have to fight other's baser nature along with your own.
Last update - the BF ended up back in the hospital. Seems like that's where he ends up all the time when he can't cope. I hope he finds the help he needs. My niece sounds like she's ready to focus on herself, I'm praying but letting go at the same time.