D.B.
I'm a mom of boys. I had all sisters, no brothers. So I feel that I have a little bit of understanding from both sides.
Here's the thing, mom. Things will never be equal between the world of girls and boys when it comes to hitting and chasing. Boys are always going to be considered "the problem" by a mom of a girl. Unless she grew up with boys or has boys, she will not accept equality for hitting and chasing. She just won't. Her daughter can do it, but the boy can't.
I will tell you straight away that if someone's kid was doing something they weren't supposed to do to my kid, I DID speak to that kid. Neither you nor anyone else will ever convince me that it was inappropriate of me to speak to a child who was hitting my kid in the head or kicking them. (You can tell that has happened to my kids, specifically!) So this idea that you should have told her that it was unacceptable to go near your son is just not going to fly in real life, mom.
Having said that, I'm glad you didn't go talk to the mom. I know you feel that you didn't support him, BUT... you would have ended up getting into an argument with her, and then she would have stopped being sweet to your son. It's NICE that she was sweet to him. I've had parents be really rude when they saw things differently than they really were.
It's best to let the school handle this. I think you are doing the right thing by talking to the principal. Explain to her that this mother thinks that just because she is a girl, she can treat a boy this way, but a boy can't do the same. I think if you approach it from this angle, asking for this mother to be told point blank that her daughter is an instigator, so that she knows that she is not blameless, hopefully the mother will get a bit of humility about her. She needs to hear it from the SCHOOL - not you. She will not believe one word of it if she hears it from another mother.
Dawn