why?? does he want to go single speed or not? imo if he's going to go 2 he might as well go 5-6, i.e. go single on the front and just keep the rear derailer/cassette. if he's REALLY set on 2 gears, then i would also think that instead of doing 2 up front, use spacers and do 2 cogs in the rear, depending on how much he cares, might not even do a derailer in the back, just the tensioner, need to switch gears? stop and switch it over.
i dunno... i just think it'd look kinda goofy to have the extra ring and derailer up front when it's so simple in the back.