If you are truly friends to both of them, they deserve your respect equally. In other words, you cannot confront the "cheater" without also speaking with the other party....and vice versa. You must either inform both or neither one.

Of course, what does this make you? Are you really being a friend or a tattle-tale? IMHO it's best to truly be a friend. Keep your mouth shut and be ready to provide support when needed. That day will come.