If he had any compassion he would have drowned the dog rather than let it live in agonizing pain.
Say what? Euthanasia, maybe, but drowning?
A final visit to the Vet's office, if called for... but drowning? No. Especially in this case, where the owner had to work at getting the dog comfortable with going into the water, so many years ago. To betray that trust would be particularly cruel.
The owner is aware that he may have to euthanize the dog, but I can't blame him for wanting to try the treatment(s) first. In a different article about it, it was indicated that by the time that picture was taken, the vet had already prescribed pain medication, but that it was early in the regimen. The owner knew he might not have more time with his dog, which was why he called his photographer friend.
Hopefully the dog will get enough relief, and if not, hopefully the owner will do the right thing.