In this situation, it's a really tough call. You could try to talk to the wife privately as well, to understand her reasons. But this has risks too. Maybe she will deny it or get angry. Another option is to anonymously send some hints to your friend so that he can start to pay attention to his wife's behavior on his own.
Well, this is a very complex and sensitive situation. You have to be very careful. If you have a close relationship with your friend, you may feel obliged to do something. However, directly interfering might make things worse. You could also consult with some other mutual friends who are more experienced in handling such matters and see what they think. Maybe together you can come up with a more appropriate way to handle it.