Changing passenger name on ticket for return leg?
 

For medical reasons my dad will not be able to fly for the remainder of 2013 and possibly for a while after that, yet he still has a return ticket to SIN in F left to use. Is there any chance to change the actual passenger flying so that I could use his ticket instead?

No change of name as far I as know. You could cancel and get a partial refund (in best scenario, deduct the one way fare from the fare paid -in some situations the one way fare is as much or more than the return fare).