My best friend is homewrecking

Ive been friends with this girl for over 12 years, we’re both nearly adults now, and we’ve always been very close. She’s always struggled with her insecurities with her weight and is open to me about her feelings and depression. I’d like to say it’s gotten much better, she’s on pills for it and she genuinely seems happy now. But her love life is a total mess and its effecting us :(

We’re both on different pages with our social status and the men were interested in, it’s gotten to the point where she’s getting used by a boy who’s in a relationship, and she’s okay with it. It hurts me so much that it’s gotten to the point she’s willing to help someone destroy their relationship. I know she struggles with finding boys who aren’t trying to screw her over, but this is what’s happening, but to make it worse he’s taken. She said she would stop sleeping with him after the first time, and now she says to me it doesn’t bother her.

It makes me mad because I know she would be devastated and say that if someone did this to her she’d never trust a man again. Yet shes doing this to someone else. But I don’t know how I can be friends with someone who is intentionally homewrecking. Any opinions? Any would help

If this were me, I would want to know so badly if my bf was cheating on me, and the fact I know but can’t do anything is tearing me up (I don’t know anything about the other girl so can’t tell her)