Should i tell? Your being Cheated on.

Okay so this guy I've known sense 2012 . me and him have been having sex sense I was 17 turning 18. I am now 20 almost 21. I dont mess with him at all anymore. And haven't for 4 months now or longer. Well I met this guy thru my ex best friend he was her neighbor. She was also having sex with him the whole time. I didn't know. Then I find out he has also been having sex with my ex best friends mom. The whole time also. This guy has been married sense September of 2014 and I've even done the deed with him after he got married. And so has the exbestfriend and her mom. His wife has been oblivious for the longest time! Because the guy and his wife have been together sense 2012 but recently married and he is still cheating. I want to tell her everything but I'm not sure how to let the cat out of the bag.