Thanks for sharing the story. My previous lab (Smokes) had a similar obsession with tennis balls.
At age 12.5, he had some serious health issues (laryngeal paralysis and megaesophagus, resulting in aspiration pneumonia) so I took him to Auburn U. Vet school. They tried antibiotics/fluids by IV but he was not getting any better. He had gone from 90 lbs to about 60 lbs in a few weeks. After a few days, he was sent home with me so he could die at home (was told he had a day or two at most).
I talked with my regular vet, and she gave me oral antibiotics to try for the pneumonia so I used a blender to prepare his food with the meds (due to the megaesophagus he could not eat solid food) and fed him with a syringe. After a month of the oral meds and eating blended food/baby food, he got well enough to take daily walks with me and even chase a tennis ball. I think being at home was what made the difference.
Over the next few months, he gained much of the weight back and had another good year before developing bone cancer (front leg). So, back to Auburn Vet school (they were amazed that he was still alive) for radiation treatment. After a few treatments with no improvement, he got to where he could not walk at all due to the pain. It was about 3 months after diagnosis of the cancer that the vet came out to my house on "the day". He got his favorite treats and wagged his tail to the very end. I know he had a good life of almost 14 years but it seemed like such a short time.
It was about a year and a half (6 months ago) before I adopted my current lab, Coal, who is almost 6 yrs old. They are great companions but it is tough to let them go.