First off I guess he just wants to mess everything up in his life. Not only with you but with his best friend.
You need to all sit down and talk about this. His best friend should know about it and he should know that everyone is on to him.
This really sucks for you and the best friend. How could someone that loves you and is best friends with this other guy do this to any of you.
This is the perfect jerry Springer show

i am only kidding!
Maybe its time that he has no contact with his best friend or his wife that way you can work on your marriage.
Has he ever done anything like this to you before? I would just let it be known that if he wants her then he needs to leave you alone but if he wants to work on your marriage then he needs to not talk to her anymore. My guess is that his best friend isnt going to want anything to do with him anymore after he finds out about this.