As a serious answer. I wouldn't say a thing. If he doesn't know you're there yet, leave. Give him three days to tell you. Then if he doesn't, tell him how you feel about the situation. Hear what he has to say. Then dump him. It doesn't matter what he says. If he tries to protect himself he's lieing and you know you can't trust him. If he bluntly comes out with the truth calmly say "Okay" and leave. Don't speak to him again unless the situation demands it, but be brief and specific and (most of all) monotone. Don't give him the pleasure. If he confronts you about your harsh atttude then slap him in the face, then scream so strongly at him that his teeth shoot down his throat. Tell him how you feel (again) and go back to it. He should learn his lesson. It's like rubbing your dog's nose in the carpet where it just pissed, harsh but it works. (Sorry not to say ur piss or anything but you know what I mean sweetie! :3 )
I would have to kiss her to see how great she is...nah j.k. I would slap the b*tch, dump his ass and hopefully could kick his balls, and jump for joy!!
Then break up.
And finally when he was hopeless and dead inside, I let him die...huahua