This is very tricky. She has to want to leave, you can’t just “make” her. Maybe a sit down, if you can manage it, with her one on one. Not to beg her to leave, but make your points about why you think he’s abusive. If she still doesn’t want to listen, just make sure she knows ur always there for her, no matter what. Men like that will isolate their victims, make sure she knows ur ALWAYS there for her. Outside of that, maybe let her know some places to call for help if she ever changes her mind.
One thing I suggest, as someone with experience, is don’t try to immediately accuse or insult the bf. People in this situation can defend their abuser because they think it’s what they deserve or just what they are comfortable with atm. The thing that actually ended up helping me was listening to true crime and hearing real stories of other people and how it escalated for them. It’s hard to acknowledge you’re in an unsafe place without hearing someone who is/was in the same place
I cannot under any circumstances recommend that you kill him because that would be illegal. Also, it would just be so sad if an abuser died. Save evidence. Hide in the closet with a camera if you have to. Photograph any and all injuries that result from the abuse. Evidence is a tool. I cannot legally recommend blackmail. Take evidence to police first. However, if police dismiss your case, you can tell the dude to back off or lose his job/friends/family.
Is there anyway for you or anyone else to set up a room/space for her? If she really feels like she had no where to go or stay, showing her that she does have a spot, even if it’s just a cleared off couch. That might make her feel more welcome if that’s really the hang up. It could also just be an excuse so she might flip to another one, but it might make her feel more welcome to come back later.