Discipline Issues

Age is a big factor here. But regardless of age, if what he did was an accident then all that is necessary is a good talk about safety and empathy towards others. If what he did was an accident then punishing him for having an accident doesn't make any sense. Talking to him about the dangers of throwing rocks near others does. (And maybe tlaking about treating others the way you would like them to treat you.) Again, a lot depends on his age.
Good luck.

you do not say how old he is. That makes a big difference

A. the age of your child is important to note. If he is between 4-10, I would first ask him to explain what happened. Then we would talk about the seriousness of what happened even if it was unintentional. I would say what could have happened and how rocks are not for throwing at anything or people. They can hurt people really bad or damage property. He is never allowed to use rocks and I'd make him apologize to the child. If he is older I'd be more stern and discipline would be sterner too. If younger I'd just simply say no rock throwing at all and make him say he is sorry to the other child. This is assuming in all cases that he never has done this before. I would try to be serious but try to control my own anger. He will hear you better.

I just want to point out to other responders that she DID give her son's age. He is 5.