I would be if he had been throwing waterballoons at her first! I would've told him don't be surprised at retaliation. Which is what I have to tell my 3 boys all the time anyway -
"Mom, he hit me."
"Did you do something first?"
"No, well, I only kicked him."
"Well then why are you surprised? If you don't want your brother to hit you, don't kick him. And if you kick him, don't be surprised when he hits you."
I raised 3 boys myself (we all survived!). I know that boys are not always prone to giving you exactly the truth if they think it will keep them out of trouble. Somehow I don't think little girls are any different. I am not suggesting that the girl, from the story presented, deserved to go to jail. What I am suggesting is, we may not have all of the facts.
In addition, if it were my little girl, I would NOT encourage her to throw rocks at someone who was annoying her (water balloons are an annoyance, not life threatening assault). You could do much more harm with a rock (she appears to be a pretty good shot). A rock to the eye could do permament damage.
susie