It’s a difficult position for us because it’s hard not to be kind to him.

Actually, given this guys MO, I'd think telling him he f****d up and to piss off would be rather easy. People need to take a no holds bared view when dealing with cheaters, male or female.

When your dealing with someone who cheats, ask yourself the following question. If this person would screw over their partner, how can you trust them?