I have to say that I personally have been in that horse owner's situation. Three years ago my mare injured her right front leg. Over the course of the next year after the injury we had 4 vets involved and I even took her to Michigan state university for x-rays and advice.
She was misdiagnosed over and over and I came to the conclusion now that we have laid her to rest that none of the vets knew what the problem was. I was told stall rest, stall rest, and more stall rest. I was told that she should recover from the injury, but she never did. After about a year of stall rest and a small turn out paddock I took her back to Michigan state. They took more x-rays and compared them with those from 6-7 months prior. They were shocked to find that she had developed severe laminitis in her left front leg and there was nothing they could do for her.
Had I known then what I know now I may have approached the vets a little differently and put more thought into putting her down earlier. I watited until a vet recommended we put her down. She was on three legs as well, but I was always told she should recover. It was tough to let her go, but I would have sooner had one of the vets even hinted that we should consider this...which none did until the end.