Does your friend use Photoshop a lot without rebooting every day or so? I know some of the people at the job I previously worked out would find that their Scratch disks would balloon in size... I don't think there was any actual way to check how much space was allocated for it, but the issue was found to be that if Photoshop crashed, the scratch disk wouldn't get emptied and it wouldn't clear out and start anew when they relaunched Photoshop, it would start a new scratch space instead. So when people left their machines on for weeks they'd suddenly have issues doing minor things because this space never got emptied.
I guess in any case, the same issue could happen with a multitude of programs. Although OS X seems to be much, much, better than Windows at keeping space clear, it's not perfect, so properly closing out of applications and restarting from time to time may help.
