How to tell your best friend she's not doing the right thing?

My best friend is going out with someone that likes me, I don't want to watch her get hurt and now she's mad at me because I told her that I didn't like her and him because he is just using her to brake up me and my boyfriend so he can go out with me, HELLP!! WHAT SHOULD I DOHow to tell your best friend she's not doing the right thing?
well first of all you shouldn't have gotten involved. I understand that she is your best friend but you should have let her find out on her own because now she might think that your jealous of her or something like. My advice to you is not to do anything because sooner or later she is gonna realize that he is only playing with her and she will apologize if she is truly your friend.How to tell your best friend she's not doing the right thing?
Let her work it out. Don't talk to her while she's mad. If she is a good friend she will get over it. I don't think that guy is worth you or your friend. If you DO go out with him, you could lose a life-long friend for a 2 week old boyfriend. If a guy would do that to your friends to get to you SCREW HIM! I would hate you to if you dated him after what he did.