re: Friend confided in me that he's having an affair (updates in OP)

In college we had some friends who were married. Awesome couple from cali. I ended up working at the same place as the wife and noticed she was very friendly with a group of young 18 year old girls and the guys she hung out with...they would go out all the time after work late at night mostly without the husband.

I eventually came to know that she cheated on him with one of the dudes at least once and possibly more. I was torn for a long time as to what to do because the guy was being made a fool of. I ended up telling him my suspicions one night when he brought it up. He basically shut me off as a friend and went in denial. They ended up divorced 18 months later when he learned first hand what she was doing.