It's so hurtful to find out that our significant other has cheated. I can relate...my husband cheated a bunch of times...I haven't caught him doing it physically, but I have caught him doing it emotionally; and to me, whether it's physical or emotional...they're pretty much the same thing. The fact that your gf is acting resentful towards you because of your ultimatum and having to cut ties with her "friend" is something you should be concerned about. It seems like she has feelings for him still...and she IS emotionally attached to him, HENCE, the reason why she is acting that way. This is a guy she cheated with for God's sakes...she should stay away from him if she wants to keep you around...and if she indeed really loves you with all her heart and being, then, she should let him go completely and that means -- no contact with him whatsoever. So, she needs to be honest with herself -- and with you...if she wants this guy in her life -- then she needs to let you go...and you should definitely not stick around.