My wife is having an an affair with my married real good friend, what do I do?

My wife and I filed for divorce after 15 years of marriage as a last resort to attempt to force our hands. "Fix it or get out". I just discovered that she is having an affair with a life long, married, very close friend of mine. I'm the only person who know and I feel terribly betrayed by both of them. What should I do about, his wife, the info I have and how do I deal with the betrayal myself? It's the worst emotion I've ever experienced and I'm trying to get a grip. This guy has been around me every step of the way with advice and input right down to helping me with the house I just bought. Somebody please give me some good advice.