*return smack across the back of head* (never done it before but it feels good)
that was for getting into this shitty mess in the first place. the other bros have given good analysis of the situation and provided good advice. I just have to hammer it in your head.
1. Your friend should know the background of this new wife. If he doesn't you should try to make him understand and tell him about her potential devious motives and consequences.
2. Have a meeting with your friend and all other bros involved and persuade him to stop it using reasons above
3. If he doesn't listen then tell him the truth that you guys banged her. If he can swallow this, tell him the danger of her wife ratting the other bros out. If he is still undeterred, then tell him that all family and friends gathering in the future will not include him or his wife.