swama, I agree that you need to talk to him and tell him this is not to be tolerated. Yes, he is behaving completely inappropriately smacking an adult woman in the butt and laughing.
On the other hand, I suspect your friend is over-reacting. I had to actually google "eve-teased" and i think your friend is making way too much of this. Maybe it's time to retrain your son how to act, and also dismiss this woman from your circle of friends?
It is inapropiate for him to do that, so have a talk with him about it , maybe also tell the friend you do not like him behaving like that, so she should tell him No if he does it, otherwise he will be getting the message its okay ...