This question comes up rather often. In a situation like this, just call CO and cancel the return portion of the ticket after the outbound itinerary is flown. You will then have one year to use the return portion of the ticket (plus change fee and difference in fare, of course). Everybody wins: the passenger does not engage in "throwaway ticketing" and preserves the return segments, should they choose to use them; and CO gets to sell the seats that otherwise would be allocated to the no-showing passenger.