Someone please help me decide

So I was dumped from a 4 year relationship and just found out today that she slept with one of my friends. She did it in the spur of the moment but my friend did it to spite me because 4 years ago the dated for 2 weeks and that was his first girlfriend. Now I don’t know to keep him as a friend or let him out of my life. On top of that I think she’s gonna try to talk to me about it soon but I can’t seem to want either options I’m so confused and angry please anyone talk to me about what to do