If the ticket was issued in your wife's name you cannot sell it for use by someone else. You can cancel and redeposit the miles into your account, for a fee ranging from zero (for Platinum elites) to $75 (for non elites) if it was booked after December 15, 2009, higher fees if booked before that date.

If you get a letter from her doctor indicating that she is not to fly for health reasons you could try to get the redeposit done without a fee, but it's not a sure thing - you'd have to specifically request it and hope for the best.