What should I do in this situation?...who's right?

okay, so about a month ago my girlfriend (of 2 years, living together for 1) and myself get into a pretty bad fight. she ends up saying she wants to take a "break" and get some time apart. she gave me no time frame from the get-go of how long she needed, etc. and for the record i never wanted the break to happen...about a month into it she starts texting me saying she misses me and everything. she did tell me that she kissed another guy when on this break and i told her i did the same thing...one thing leads to another and we hang out at my house. so things get intimate, but something feels wrong..so i ask her if she did more with the guy..it turned out she slept with him in that short time we were apart. she was devastated...saying it was the biggest mistake of her life. and i didn't want to lose her again so i tell her everything is going to be okay... we're still hanging out and we will probably end up getting back together, but she wants to stay friends with this guy that all that happened with... i told her she would just be hurting me and i didn't want that to happen..she says she isn't going to just stop being friends with someone for no reason...apparently my reason wasn't good enough... am i wrong in this situation to ask her to stop being friends with this guy?...