Ok, my first question is does he have two physical drives?
Or is it a single drive that is just partitioned into a C: and a D:
You won't get much done in BIOS.
If he has two physical drives, then too bad, there is nothing he can really do except maybe reinstall Windows on the larger one.
If he has one drive that is partitioned, then download GParted ISO and make a bootable disc. Boot from the disc.
Then you will be able to re size the partitions acording to your liking. So you can make the C: bigger, and D: smaller. Or simply merge them to create a single C:
You can look up how to do that on GParted website, then have good tutorials.
http://gparted.sourceforge.net/