Simple question: You have two married friends. One starts cheating. Do you tell the other?

[Xithras]

This popped up last weekend and has me torn. I have two friends who have been married about 10 years and have two young daughters. My wife and I have been friends with them for about four years now. Without getting into the gory details, something happened this weekend that revealed that the wife is not only having a full-on sexual relationship with a guy who isn't her husband, but that it may have been going on for quite a while.

Would you tell the husband? I know him pretty well, and he's not the kind of guy who would be OK with this or look the other way, so I assume that he's in the dark about it.

If you were married and your spouse was cheating, would you want your friends to tell you if they found out?