The first thing I would do is give him/her the dressing down of their lives and tell them that if I ever see them do it again I would report them to the police and get get them charged with animal cruelty which carries a substantial fine, a prison sentence or both.
I had an incident a while ago when I saw a man hitting his dog with a stick. I was so incensed that I grabbed the stick from him and told him if he ever did it again and I saw it I would beat him with the stick to see how he liked it.
Against the law I know but why should a brute inflict pain on a defenceless animal.