Cheater cheater....

So my husband works with this guy who has been meeting up with this skanky chick that he met a week ago with intentions to hook up with her. The guy has been married for almost 20 years and is married to a family friend of mine. We don't talk on a normal basis but I've known her pretty much my entire life. They have 3 kids together and seemed pretty happy. I'm torn. I want to tell her but I don't want my husband to be mad at me or the guy to lash out at my husband... What do I do?!