My FP's best friend ruined our relationship

I just cut off my fp (or attempted to) because his best friend has done repeated concerning behaviours and he continues to defend her. It hurts, a lot. I feel like I have a lot of sympathy for her because a lot of her behaviours are stuff I know I'd feel if I felt my fp was abandoning. But it started when she threatened to unalive herself after she learned he was staying at a guesthouse with me for a week. It was the first time I'd ever seen him cry. And I encouraged him to tell her to seek help even though what she did still didn't sit right with me. But it's just been more things like that ever since then (that was October) and he told me yesterday that they're going on a 3 month trip and I it feels like he's just been enabling her codependent, toxic nature and it's never going to change. He told her to move out (they were roommates) after the guesthouse incident and I was surprised he was actually able to set that boundary. But when it came down to it, he started feeling really guilty when she said they'd never be friends again and that kind of stuff, and I guess he got scared of losing her. He instead basically made me make the decision by asking what happens to us if she stays, and I said there would be no us because it would be he is allowing her behaviour. So he told her to move out. But I didn't like that the decision fell to me, like I was the one making her move out.

Then there was a time he and I hung out, she called him (he's very cagey when she calls) and I heard her say 'fuck you' loud and clear onn the phone when she learned he was hanging out with me. That was like 2 weeks ago. It baffles me that he doesn't tell her that's not okay, he just lets her say that to him. And he lets that behaviour continue and I've tried to tell him he needs to set boundaries but he'd rather listen to her I guess. So I told him today that I'm done. And I'm terrified because I feel like I need him. But I'm exhausted. And I don't think he'll ever see what she's doing as worrisome. And I feel hypocritical because, I don't like the emotional manipulation or guilt tripping she's done to him but I've probably done that too. Even after I told him I can't stay in this dynamic, he said "technically she didn't actually threaten you" and honestly that just twisted the knife. I don't want to lose him at all. But I can't let her shit continue to run his and my life. I just can't believe how blind he is. He's had problems when I do things far less destabilizing than her. But I guess I couldn't crack their 14+ years of codependency. So I just feel kind of adrift. I've been heavily reliant on him for emotional support and I don't know what to do with out it. I don't even want to stick to the decision of cutting him off. I don't know what to do. Amidst it all I feel like I'm overreacting.