how is he shareef if he is having an affair?
ok here is my advice (a lot of u may not agree with this)
he doesnt need to confess anything to his wife. If he wants to continue being married to her he needs to stop what he is doing immediately, repent to Allah SWT for what he has done and spend the rest of his life keeping his wife happy by behaving the way a husband should....which if he is Muslim would be in accordance to the sunnah.
Confessing will just cause her to either leave him or carry ill feelings towards him while they are married. she will have a hard time trusting him again which will eventually lead to a failed and/or unhappy marriage.