I think the yard owner talking to the parents is the best outcome. Once everybody realises how she is behaving she will hopefully have a closer eye kept on her.
She sounds like a very unhappy child, poor child and poor pony! Hopefully, now the YO and parent is aware of the problem this will be stopped and parent will begin to realise that the child is very troubled and needs help. I am not defending the child's behaviour but IME this is a sign that something is wrong.