Explaining a problem

I’ve had a problem recently. Two of my friends started to go out. The boy is in love but the girl flirts with another boy.
I’m worried because If I don’t do nothing, maybe one of my friends will be annoyed with me and I will don’t have the fault. I think the best idea is talking with my friend girl and he doesn’t flirt with nobody, but she would get angry because I can’t say what can do or can’t do. Another option can be to talk with the boy and say to finish the relationship but he would get annoyed because he will think that I like the girl.
Please, I’d really need some advice because if I don’t do something soon, I’ll probably have problems with my friend.