Not really, but that's because I'm only a lonely goddamn thirteen year old.
Personally, though? I don't see why you'd not talk to the guy. People have feelings, theirs just happened to be a bit more tangled up in your own. Even if he was sure your ex-girlfriend would be cheating on you, so what? Life isn't like in the movies or tv-shows, where you put your friends above all else. Affection, even simple lust, can overcome anything if strong enough. What if he loves her? Would that still warrant your anger?
Ach, sorry. I get tangled up in tangents a bit too often sometimes. Still though, something to think about.