Is it fair to end a friendship over a friend’s relationship?

I’m sorry if this is not the right subreddit, I’m not sure where else to post this, and I also apologise in advance as it might get a bit long.

I have been friends with this girl for a few years, and overall it’s been okay, we have some issues, but it’s not what I want to talk about today.

So about a year ago this girl got into a relationship with a guy, I never liked him, but it’s her life and her choice, so I just minded my own business. For a bit of context, he’s Albanian, and I am not saying they are all bad by any means, but he displays the typical toxic behaviours associated with those men.

She’d routinely come to me with their issues, and I’d try to give her advice, the same she would and previously has given to me to me when the roles were reversed, but she’d never listen, and stayed in the cycle. After a while it did start getting very tiring, but I still thought to myself I am her friend and I’d want someone there for me if I was in this situation.

NOW, where the real issue for me began is, she tried to get the two of us closer, and for her sake I participated and always tried to be nice to him. One night the three of us went out together, and at the end of the night he became verbally abusive and and a little physical towards me. I stood my ground and he absolutely hated it, as in his eyes I am but just a woman and he’s a man, so I should just take it.
I stopped making absolutely any effort towards him from that point onward and refused to meet him again, which she was annoyed about? She kept telling me to just get over it for her sake, and apologise to him for not taking his shit that came out of literally nowhere. I still said no, and she couldn’t force me, even though she continued bringing it up.

Now this guy is in prison, for some reason she’s stayed with him 🤷‍♀️ He keeps being toxic and controlling to her even from inside, and routinely basically daily tells her how much he hates me, calls me different names, tells her she shouldn’t be friends with me as I am a bad influence (it’s honestly very opposite actually) and more.
On numerous occasions he’s also threatened her with literal murder? The guy is absolutely insane, and I am worried both for her and honestly my own safety knowing how much of a vendetta he has against me.

I cannot force her to leave him, but I have spoken to her about my concerns previously. She just waves it off and laughs at me saying he is all talk and won’t do anything, but he is quite literally a criminal and now surrounded by a bunch of other criminals, which I don’t believe will do him any good either. He is due to get out in a few months, and he knows where I live. I am genuinely very uncomfortable with all of this, and don’t see any way where I can permanently cut him out of my life, without cutting her out as well, as she is the only link.

I feel terrible about myself for even thinking about this, because he does put her through a lot, and she has been there for me when I was in a shit relationship too. The difference is my ex never had a problem with her, and neither I or her ever felt threatened by him.

What would other people do in my situation? I’m thankful for any insight anyone can give me, I am personally struggling to separate my heart and my brain when it comes to this. Willing to answer any additional questions people have too, I just know I’ve already spoke for ages. Thank you in advance! 🙏