You can talk about that as much as you want to someone, but really, your friend must be seeing through rose-tinted glasses right now, and it's very hard to get someone out of that trance until they get completely screwed over - I know from personal experience, and i'm sure there's others reading this thread that have experienced this too. Just be there when it all goes to crap for him, which I imagine it probably will, if what you say about this girl is true. Once a cheat, always a cheat.