Regardless, if you can't make the change, cancel the reservation and consult the fare rules (post the fare basis code and airline here if you want my help) for reticketing information and fees. Be aware, generally, that you cancel a ticketed reservation up to midnight of the day of flight at the departure airport. After that, a non-refundable ticket goes to zero status and has no residual value. Get your info now, but wait as long as possible to cancel.
If the ticket is refundable, simply have your friend get a refund to orignal form of payment. There are other, less savory strategies we use in manipulating cancellations and refunds, but the above are the "official" ones
Lastly, if your friend made the reservation through a TA (travel agent), IME he'll need to change/cancel through them. They "own" the reservation.
Good luck and hope he gets well soon!