WTF are you doing, and all your shithead supporters here? Do you people have spines or is it all dick holding you up straight?
You should be upset with this so called friend of yours for doing something so disgustingly lowlife to you AND her boyfriend. What kind of relationship do you expect the two of you to have, based on some fucked up drama like this? It's going to totally suck and you should realize it, your friendship will be ruined. Are you really so blind because there are emotions flying around? Lay away your emotions, be a man, confront this girl, tell her it won't happen and find someone else.
I've seen great, nice guys getting destroyed over shit like this. Motherfuckers, you NEVER mess with a guys woman, or you deserve all the fucking pain in the world. I don't care if you don't give a shit about your girlfriends and take this with a shrug, I'm speaking on behalf of those who do care, and seriously OP are you not one of them? Be a friend to this woman and do the right thing.