Cat Behavior Concern?

I have a friend who had taken her persian cat to the vet and ever since then the cat seldom gets out of the litter box. The cat is an adult cat and has been to the vet before. He just has never slept and layed in his litter box like he is now. He does comes out for awhile to eat but, goes right back in the litter box afterwards. This cat is the only cat in the house and there are no children around him. He also has 2 litter boxes that are cleaned out regularly. Any ideas?