How do you tell someone their partner is cheating?

Tell your friend, man. He deserves to know.

He'll find out eventually, or he'll have no balls for the rest of his life. In both cases, if you say nothing, you're a shitty friend. I know I would not trust my best friend any more if he saw my girlfriend cheating and didn't tell me. Unless, of course, she was a good friend of his as well, in which case I'd see his dilemma... but she already hates you.

What the fuck are you waiting for?

Jam it back in, in the dark.

Be upfront with him. Tell him exactly what you saw, no more or less. He might need a couple of days to formulate a rational response, and unfortunately you might get the very irrational one. Expect it, don't fight back. Try to balance any of his irrationality with as much calm as you can muster. Don't take any bait, don't start any fights, and if he tries, just walk away.

A group would put him on the spot and make him feel like more of a chump. Don't wait around for serendipity to bring you to the same bar as bitch and her fuckbuddy(ies) again so you can get pics. If he needs proof, give him the names of other people who know.

Don't listen to these pussies. Keeping your mouth shut is only an option if you're a gutless coward and a shitty friend. You don't seem like either.

There's nowhere I can't reach.

You cowardly, weak-willed motherfuckers. Maybe if you valued your friends over the instant gratification of your their momentary approval, you'd forge stronger, healthier friendships.

Man, I really hope you aren't buying into this selfish bullshit. The longer you sit on this, the more he's gonna hold it against you. If he's worth your continued friendship, he'll understand that you're being a good friend, and if he does get upset, he won't direct his anger toward you for very long.

This thing is sticky, and I don't like it. I don't appreciate it.