I think you are missing the point. Even if she wanted a platonic relationship with the guy, still what they did wasn't right. It has nothing to do with cheating on her BF. It's the principle. And the OP was pissed on her friend not on the guy, maybe because she knew her for a relatively long time.

Anyway, it seems this will drag us to some arguing. The bottom line is that the OP has a problem, and expected us to help and support her. I'm not sure if she found either. People make mistakes, we all make mistakes. So, please don't tell me what's right and wrong, try to help her if you can. I'm out.