Actually, you don't know why she was mean. And it doesn't matter. I suggest telling your boy that if someone's mean, then stay away from them. If they corner you, kick 'em down and keep 'em down. I also suggest, if another kid's parents can't keep reign of them (/teach them ignorance), for you to tell the kid to go and play by themself. If the father has a problem with that, the police can be very handy. If he doesn't like that and corners you, kick him down, and keep him down. (If it's a female parent, slightly different physical effort is required.)