If the marriage is "all on you" then I guess you'll have to take the lead.

Sit it him down - let him know what you know - let him know that you will be monitoring his email going forward (indefinately) probably voicemails too. Have him call this lady on speaker phone and let him tell her that what they were doing was inappropriate.

I guess I'm a little opinionated on this one sorry if I've been too blunt or strong. I hate it very much when men like this act like this as it gives Men a bad name. Though I would suggest the same to a man if he suspected the wife of the same.