Should I tell his wife?

[Chupacabras]

Minding your own business is always the best policy. At least, for anyone that isn't a member of your family.

[Moe]

Maybe this has to do with my hatred of cheaters but would tell. People who cheat are the lowest form of life form just step back and look at their morality.
Lets see he's effecting you, his wife, his kids, probably his extended family and many more.
I would let him invite you out and you invite his wife to the same place.

[Gribble]

More people cheat than any of you know or care to accept.

It's an extremely common practice and you're kidding yourself if you think otherwise.

Mind your own. Let others sink or swim without your interference.

[Wiredbomb0]

It would be wrong not to tell her, simply because she shouldn't deserve to be with a cheater (no one does). I have no respect for cheaters, what-so-ever, and frankly, he chose to make that mistake, and so he deserves to face the consequences, whatever they may be (jail anyone? lol)

However...

On the other hand, telling her might cause trouble with the couple, which, in-turn, may (more-then-likely) will affect the children, and I know from my friends, having split parents is certainly not a good thing.

Think about the children! ;(