That's a tough one. Knee jerk reaction is to say, nobody should be hitting your horse. Farriers though have to work in a very dangerous area and it's not their job to train the horse. While she might not be using the best method to keep the horse from pulling back, she sounds like she's doing what's necessary to keep herself from getting kicked.
Like you said, a good farrier is hard to come by. I finally found one worth his weight in gold, and my horse chose his first visit to act like a butthead. He repeatedly pulled his hind legs back and really ticked him off. I spent the next six weeks working with him on ground manners. Everyday I would pick up his back legs and just hold them. I didn't put them down until he was completely relaxed. I also did some other respect related things such as yielding his hind quarters, sending him through tight spaces and backing up all on a longe line. Next time my farrier came out, he stood like a champ.
My point is, meet her half way. Try to provide her with a more compliant horse next time around. If you absolutely object to her hitting him, maybe ask that she simply hold the leg until he relaxes it. If you do the exercises in advance, you're horse should respond to it quickly and maybe she'll be good with that compromise. That's all I've got.
You just have to see your distance...you don't have to like it.