I find it difficult to believe that this dog would have seen the vet without it being known what had happened, it's a sad story and people talk - it's human nature. Don't know how this vet can sleep at night, I really don't :-(
My guess is that the shelter was full and there were much younger dogs looking for homes, ones without arthritis :-(
Years ago, back in the 80s, there was a huge stink when a young Burmese cat was found and taken to breed rescue. The breed rescue didn't have space that day but as a cat was being collected 2 days later they knew there would be space. It was arranged that the cat would be housed by a well known animal charity for the 2 days and bed and board was paid in advance. When breed rescue went to collect the cat as arranged it had been PTS! They claimed the vet had pronounced the cat too thin to save! This was after breed rescue had assessed the cat and found it healthy (young Burmese do look quite slim but weigh heavy) It transpired that the owner had been found and then had to learn that their cat had been PTS

An official warning went out to all breed club members against trusting animal welfare charities other than breed rescue.