I ended up with a kidney stone just prior to a nonrefundable flight to Paris. International fares are fully refundable- so I was told- and they did cheerfully refund the full amount with a doctors note indicating I was unable to fly. On domestic, with a note, I was told that it is not refundable but you can use the funds for future flying within a year. I do not know how it affects other passengers than the actual sick person.
I would call after you have the letter and see.