OMG. I do hope this doesn't set Jack back for long.
In all honesty, I think I would have reacted the same way, and then got home and wish I'd had a go at him. I don't know. I guess you never know until you are unfortunately in that situation.
I was out with Marlee a few weeks back, we where heading home when this man and his dog came along, Marlee was off the lead (his dog was on), Marlee went to say hello and so did his dog, and he whacked his dog across his back - very hard. I immediately called Marlee back, thinking he might do the same to my dog. I couldn't believe it. Some people... *sighs*