And you need to take some responsibility young man for letting that shitty of a situation come to fruition. Not a good feeling is it? Don't go through life like this. Grow a backbone and don't let some bitch pull this shit on you. You are headed for a life of misery and being walked all over.
First thing to do is tell your friends GF and fuck his relationship up.
Unless his friends gf is a very, very good friend of op's it would be 100% unbosslike to get involved. It would actually be a bitter, butthurt, bitchmade move.