It depends if you tell her or not.
1)Does the wife trusts you?
2) you don't have any proof if you tell her and may be later her husband tell her that you are lying or prove it what will you do?
3)Its not your bussiness
4)Morally its good to tell the wife but if you think about result it will ruin one khandaan, one family
5)Jo kisi kaa parda rakhta hai ALLAH us ka rakhta hai its not court where you have to be a witness so think about it first before telling
6) According to my experience don't get your hands dirty in other's personal matters but on the other side stop listening to your friend. Whenever he tells you something you should say can we talk something else or I don't wanna hear you because I am not a betrayer.
I know its his wife right to know about cheating but let her figure out herself that whats going wrong in their relationship. Because sometimes husband wife forrgive eat other and then they will blame you that you didn't try etc etc.But the point is think before telling the wife.All pros and cons