I wouldn't blame you if you did.
It will be wrong if you do it. I think you can read that as feeling angry. Sounds like you are protective of your friend and you are reacting to how he has treated her. I better use of that emotional energy would be to show empathy to your friend (you probably already have), and if you can calm yourself down and you know him, perhaps tell him that his behaviour is inappropriate and immoral.
There are a lot of jerks out there. Unfortunately it is a fact of life, just the same as there are people who want to take our money off us.....
However, take heart, the morals that we live by help define us.
You have to ask yourself that. You also have to ask if its your business.
Nope, just as long as you don't follow through on that desire.