E.Z.
Hi D.,
This sounds like an awkward situation, but your primary responsibility is to protect your son. He absolutely has to know that you will never allow someone to hurt him--especially while you are present. This little girl's behavior is totally unacceptable, even if she's nice the rest of the time. You may have to have an awkward conversation with your friend and let her know that you just can't allow your son to be manhandled, and if you both can't find a solution perhaps it's best that there be fewer playdates until her daughter is past the stage of physically acting out. I know it will be hard, but in the long term, you are teaching your son how to live and be in the world. And if he gets the sense that it's okay for someone to be physically hurtful toward him and adults don't take appropriate action to stop it, imagine the long-term consequences of that. I know it seems like a small--although troubling--thing at the moment, but it has the potential to create a bigger problem later. And in reference to the girl, what is she learning from this? That she can physically hurt people while adults watch as long as she is nice the rest of the time. Her mom is doing her no favors.
I hope this works out; it certainly sounds like a difficult situation.
Best,
E