You could always do it anonymously...telling his W, I mean.
I agree with telling his wife, anonymously if necessary. I think anyone would want to know if their spouse were cheating on them. Predators should have to experience the consequences of their actions.
I also second what Sandi said. You would only have leverage to bar your wife from the party if she were living with you. Since you guys are separate, do as best as you can with trying to put the whole thing out of your mind.
WAW Using God Me-43 W-40 M-14 S-11 S-9 D-7 EABomb 5/09 Separated 12/09