Nobody seems to have mentioned it so to be fair to the OP: according to the rules you have to fly all segments or risk your flights being recalculated to the sum of the one-way fares you flew. I.e. *lots*. In practice there are few, if any, reports of the airlines imposing this or other penalties. If your parents aren't interested in clocking up miles into an established miles account then they have even less to risk.