Explain to him this is how it "feels", that it is just feelings. The question he needs to ask himself is this : "Yes, I am attracted to this new person. But looking back on myself, would I have liked to have left my wife for someone new just because I'm attracted to them?"
This is a good question to ask. People get trapped in the affair and think they can't go back home safely. He also needs to know that what he is doing has a very low rate of success as a primary relation. So they break up the home with a new relationship for nothing, because they are going to end up single and hurting too.
He needs to look at the law of the averages of the position he is in.
