Believe me, those small breeds can be nasty and really get your temper going, nice as they may be at home with their families. Occasionally you get one that is extra horrible to deal with. We had a toy poodle that we had to care for that simply HATED everyone no holds barred. Taking him out for a potty run was a joke. You couldn't get him out of the cage, he would attack your gloved hand, attack your legs when you were trying to walk him. And of course, being that pissed off about everything he wasn't going to go to the bathroom for you, he just did it in his cage. But of course, we still had to walk him...
So, if he was anything like that, and the vet was under a lot of stress otherwise (family maybe?) I can see it happening. He should have taken steps to avoid this kind of reaction beforehand, and like I said, he needs to take some kind of sabbatical and get a handle on his stress problem.