I don't know how bad his symptoms were before going to the clinic. The doctor must have thought there would be some time to pass before things got to bad for him to say there was no problem.
I always thought that it was peculiar that he was in so much pain the same day he returned home.
I think that the doctor wanted him to go home and enjoy the last days he had of feeling good without the cloud of eminent death over his head.
I have known several people that underwent treatment for cancer and the doctor would tell the family that everything went well with treatment and to go home and enjoy life, and they were dead within the year.
It seems strange that they would give Big Daddy false hope of recovery, given his hatred of "mendacity". He would figure out he wasn't getting any better pretty darn quickly I'd think, the lie wouldn't give him much satisfaction for long and after the truth was revealed it could have been devastating. I don't get it either.
Years ago cancer was considered a Death sentence,and doctor did not tell their patients but they did tell the patients spouses and the cancer was dealt with accordingly ,in many cases the patients lived beyond their time allotted to them after the cancer diagnosis....Mind over Matter.... and positive Thinking ....