how to deal with "mean"

This happened quite a few months back but I still remember it and it still saddens me.

We went to a costume party. There were other kids there (2 boys and a girl). g had never met them before but she seemed willing to try since they seems to be having so much fun running around. As usual she proceeded cautiously, sort of running close to the group but not really with the group. Then a little closer and then closer. I stood nearby talking to the parents of the kids. The two boys were brothers and the girl was a friend of theirs.

As g was finally within touching distance from the group, the girl loudly declared to "her" group, "let's not play with her. " The "her" was little g. I was so crushed. Yes, I was crushed by a 3 year old's comment. I looked around to the parents. I didn't think they heard her because they were in the midst of conversation. (Now that statement does not begin to approach the textbook definition of mean but she was referring to MY BABY!!!!)

Though this seems to be one of those things that every child has to learn to deal with, I can't help the impulse to give that kid a good talking-to.

Thankfully, I don't think g heard.

I wonder though what I would do or say when the day comes when she will hear a comment like that.