How do I fix someone else's problem?

So I have a friend, a very special friend actually. Just last night, she came to my place sobbing. She was in full pain; I can really see it. She told me something about her affair to a guy I didn't even know. And the problem is she has a boyfriend, they've been together for 4 years now since that new year's party dating service. She wants me help her amend everything, that I am the only who understands her. But I'm not really sure about this. First, I can't stand on the fact that she's cheating on her bf. Secondly, she wants me to talk to her bf and end everything between them. It's not that easy! What should I do? How do I fix someone else's problem?