Back to the original topic, I want to share a similar experience that occurred with a breeder and close friend of mine. They raise purebred cattle that are as sound as they come, i.e. nothing clubby.
Last year, they sold a bull calf off one of their best cows; just after weaning at around six months. This was this particular cow's first bull calf, previously she had four heifer calves out of the same mating and they all turned out to be great show heifers. At about eight months old, and almost overnight, the bull calf became crippled. I forget the scientific name, but the vet diagnosed it with a genetic disorder (I would assume it's the same one that OP is referring to). The reason that it had never appeared in the heifers is that this disorder is much more prevalent in males.
Unfortunately, by the time this was discovered, the cow was already bred back to the same bull. As luck would have it, she had another bull calf back in the fall. The calf was great, and completely sound and defect-free when it was born. They decided they would keep him and feed him out to see if the disorder surfaced again. Again, shortly after weaning, the calf became crippled to the point that he had to be put down.
I saw a few people mention that the breeder should do nothing and that the problem is the buyer's fault for not knowing how to select cattle, but I disagree if we are talking about the same disorder. It's no secret that club calves are much more prone to structural issues than most purebreds, but this disorder appears nearly overnight. After seeing it firsthand, I can tell you that it is completely undetectable until it happens. This is NOT to be confused with a poorly structured calf that begins to buckle and blow apart as it gets big and is being pushed hard on feed.
The breeder I am referring to felt bad that the sound calf they sold became crippled. Obviously if they had discovered the disorder, they would have never sold or attempted to sell the calf. The buyer also understood what a bizarre situation it was, so that weren't demanding a new calf for free. In the end, the breeder offered them a new bull at a VERY fair price. IIRC, they sold the first bull for $5,000 and then they took him back and sold the buyer another young bull of equal quality for $1,000.
I think they did the right thing. I don't think the breeder necessarily has an obligation to replace the calf for free, but doing something to make it right really says a lot about what kind of breeder they are.