I've been slapped in the same manner (more than once), but had the self-constraint to not respond. Not easy, but it is a measure of one's maturity. If he can show that he learned from the incident, I can see him being a model citizen moving forward.

I, too have been in a similar situation more than once. And I will admit that there was one time I did hit back. Now it wasn't nearly as vicious as this and it was with an open hand, but I immediately regretted it and never did it again. Mistakes happen, good people learn from them and move on.