Leukemia is tough on a cat. Your right, he most likely had it as a kitten, bu tI am suprised they didnt test him before vaccinating him for it, but it is not that uncommon practice.
Maybe your daughter should read about it with you and go visit him with you. I have spent 10 years in a large vet hosp as a CVT and it is difficult for owners to make decisions from the outside of those walls. THey workers really watch the pets and understand their suffering and see them going downhill. When owners stop by, the pets perk up so the owner sees a "progression" but then they leave again and the pet does downhill again.
If she can she that he is not well, it might be a bit easier on her.