Fulford, I'm glad that you are very concerned about the welfare of training a horse that won't tie incorrectly. =D
I have had 3/~30+ horses in my ownership career with tie problems and two were OTTB's--typical, bc they never tie them at the track, just use a human horse holder.
IMHO patience and time will fix this. If the horse breaks the halter in the middle of your training it just takes longer to retrain them to tie. I eventually taught all 3 to tie and the last OTTB had many problems on my last day with me, but he WOULD stand tied for hours. I think what you witnessed was a horse that wasn't trained to tie and not worked to be calm while tied, so he panicked and flipped, never a good end. THAT was the owner's complete fault and not the horse's, but the horse suffered nonetheless.
I really think that the OP can tackle this successfully, bc many of us have had to figure out how to fix this problem.
Actually, this horse is one I have been riding and tying for 2+ years for the owner. Something spooked him as he was at a event with his owner. If he had been tied up with a leather halter, this wouldn't have happened. Sure, he may have ran away, but being in a safe environment, he wouldn't have been hit by anything.
I agree with Updownrider.
IMHO, I will never tie a horse in something that will not break, I ALWAYS trailer and tie my horse in a leather halter. Why? If a accident happens, the leather will break. Having a halter not break is just asking for problems of a horse hanging it's self.
All my own horses are great at being tied at horse shows, the trailer, etc. I have trained my PMU baby to tie with no problems, all using leather, he never broke it once, as I had trained him to yeild to pressure.
Take my advice or not, but safety first.