If he is married, consider outing him to his spouse. If they are still together, then it will really piss your wife off; but it will probably go a long way to busting the affair.
Me 36 W 40 S 13, 9, 7 ILYBNILWY 2-08 Discovered EA 3-08 Reconciled 7-08 She says she's been faking it 11-09 She wants to separate 12-09