How do you tell someone their partner is cheating?

I agree with Shin.

Unless you and your friends have something concrete to present, stay the hell out of it. It's none of your business.

And I would not use the group thing. That causes a really defensive and pushed person in the end.

Eitherway, until you can present some concrete evidence, say nothing. And heresay is heresay. Rumors and gossip are awful - don't buy into it, even if you trust your mates without question. People are prone to dramatizing events, especially when things can be mutilated beyond truth.

I'd be more likely to yell at the girl than to tell the guy. Take a cameraphone pic of her making out with some assholes or something and then blackmailing her into coming clean with her man. But I'm malicious like that.

Jam it back in, in the dark.