I'm going to call my friend pathetic and a loser after his wife cheated on him
Why would you call your friend names after having something like this happen to them.
He says he's a pathetic guy and I want to make fun of him
You should not attack your friend verbally but instead try to help him get through this troubling time. You don't need to involve yourself with his private business but be there for him when he needs you.
This is so sad.
You need to be there for your friend during this tough time. Kicking him while he is down is only going to make him resent you and make what he is going through that much tougher.