Sorry, but things happen. No sane trainer at clinton andersons level advertises killing horses, especially one in the nh catagory.
The owner sounds very uneducated.
My BO had a stallion, vet checked 100%, a mare came in for breeding, 2 days later he died in his stall. I saw him at 8pm the night before and he was found dead at 1am. No feed changes, nothing different at all and they had an autopy, which was inconslusive. Thankfully the mare owner seemed understanding and was refunded, but many a person would have looked at that situation and blamed the BO. Another example, BO took in a free filly off the track, she was a 'roarer'(diagnosed by the vet, that was the only problem he could find), with the intention of giving her some time off, then retraining her as a pleasure horse. She was put out to pasture, and a month later found dead, with no signs of struggle. Things happen with horses.