This one is a little personal for me since I had to deal with a cheating spouse.
Where as the others are right in that it is not your responsibility to tell her nor should you want the drama, you should remember it is never our responsibility to help anyone. We should help each other becuase we would like to.
The others are wrong though in that Karma will not catch up with him. Some cheaters just are never caught. It's as simple as that. Some are, some aren't. They are cheaters which by nature means they will not confess freely and will try not to be caught.
My advice to you would be of course to stop seeing this person, but also if you are able, a annonymous letter or message to the wife would be best. I know I may catch hell for suggesting that you get involved, but since my marriage fell apart because of that very reason, I did hold resentment towards those who knew, watched it happen, and turned their heads. Whether or not i have a right to be angry at those people was irrelevant, I was angry and could help feeling cheated by them too.
That's just my 2 cents.
