I was confused with that fruit thing but yeah it is grapefruit.
It lowers the metabolism of a number of medicines causing the concentration to be higher in the blood.
BTW I really doubt that the doctor could have done anything for their son. Bacterial Pneumonia is deadly without antibiotics and there weren’t any back then.
If it was the flu he could treat the symptoms but with oxygen and things like that. I don’t know if he had that with him.