I have less and less faith that UEFI BIOS can find all drives all the time .
But since you have to reboot to change drives I still cant see any problem just selecting the boot drive AT BOOT .
The drive you are not using will be powered but wont be used or even accessed unless you decide to use windows explorer
He said he didn't want to teach his family how to select the boot disk (can't say I blame him) Though the windows boot manager isn't too terrible.
Though I personally haven't had any problems with the UEFI not finding drives. What board and brand has that been a problem on?