There was absolutely NOTHING wrong with what you did. You're an understandably concerned mother. And you wanted to inform another mother about her son's behavior that seriously needed to be addressed.
I am apalled that this mother has completely abdicated her role as a parent. By not establishing boundaries, teaching her son that hitting is not acceptable, offering healthy ways for her son to let out this combative energy, she is doing nothing to correct his behavior. In fact, she is doing the exact opposite. She is only fueling the biting and hitting by her son. Sadly, she is enabling him and his destructive behavior, which is a terrible shame. In his eyes, he is doing nothing wrong. "Mom never says I can't/shouldn't do this/that--must be okay".
From the outside, I see the mother being reserved (instead of the usual friendliness) and creating distance from you as sign of embarassment and guilt. But instead of doing something about the issue, she's merely internalizing it, placing blame on others, minimizing the seriousness of the matter, thus choosing to remain in her denial of any problems caused by her son. These types of parents let their kids "run the show" so to speak. They are more afraid of what their
child will do or think about them, than ensuring they do what is necessary for their kid to be a respectable person. They don't want to be "mom" or "dad", they want to be liked and be their child's "friend".
Don't get me wrong, I love my children to pieces, but I'm fully aware that they aren't going to like me all the time, and vice versa! But as a parent, I want to be treated with respect and I want my child to treat others in the way they want to be treated. And I don't think there are many kids aspiring to being a chew toy or punching bag.
You did the right thing. The well-being of you and your child are not to be taken lightly. We can
get into trouble when we compromise things in order
to be accepted by others. But I applaud you for choosing
to be you and your child's advocate. You are worth it. :)