If you are sure, and I mean 100% sure she is cheating.. just move on. It will suck for a while, but it'll pass and you be glad you did. If you aren't 100% sure, then try to work it out somehow. Obviously what she is doing, whether its friends or more with this other guy, is hurting you. Try explaining that to her in a calm conversation. If she really cares for you, and isn't cheating, she will stop hanging out with the "friend" to spare your feelings. Once she understands that she is really hurting you, if she refuses to stop seeing him, then I'd tell her to leave on the next thing smoking.