Im with codatango on this one.
If that didn't work then i would explain to the guy that the dog didn't know what he was being beat for.
I would try to teach the guy a better why to work with the dog and offer to help him work with the dog.
If that didn't work then I would beat him over the head with my shotgun.
When I was done I would ask him if he figured out why I beat him over his head.
Some people shouldn't own dogs much less have kids.
My response would have been to invite the guy to train. Nothing positive is going to occur in a parking lot when both parties are mad. If that was his reaction, he knows nothing or next to nothing about training a dog. The dog was simply turned loose to figure it out on its own. An introduction to a regimented training program would do him and his dog a lot of good. Although knocking his teeth out would be perfectly reasonable reaction, the end result is less than satisfactory. He goes home and beats the dog and you probably end up with a police record for assault.
If he trained with me, I hope he would see the error of his ways. If not, I at least tried to improve the situation.
I don't know for sure but I would say that the reason the dog left his owner in the field and went to the truck was to get away from being beat on. I say what the guy did was wrong and it will definately come back on him.