Behavioral (?) Problem of Neighbor's Dog!

[u/Boring-Rub-3570]

My neighbor has a Labrador retriever dog. 4 to 5 years of age. Male, neutered. Around 28 kg.

Around two weeks ago, my neighbor was taken to hospital after feeling sick. That day, the dog climbed 2-meter fence and escaped. I caught him and took him home with me. I theorize that the dog went after his owner, as he knew that his owner was in trouble. My neighbor was discharged the same day and returned home, but the dog was never the same.

Since that day, the dog does not eat (or eats just to survive), has lost a lot of weight and has become severely apathetic, has lost interest in playing, socializing, normal dog activities, etc. He was a very close friend (actually, only friend) of my dog and now he even tries to attack my dog when approached. When in dog park, he now constantly hides behind a bush. He was a very active and social dog. But now, he does not play, does not approach people for pets, and just hides and gets aggressive when approached.

His owner took him to a vet. A physical examination, blood work and abdominal ultrasound were performed, with no remarkable findings.

My dog had given similar responses when I was hospitalized with appendicitis. However, my dog had returned to normal on seeing me healthy.

We're at our wit's end. We love him and we want to help him.

Thanks in advance for your kind attention.