Originally Posted by
msdumo
Does my theory hold water?
Yes, but you shouldn't need to give them a full-on bleed every time. Orient the bike in a stand such that the brake line is constantly running uphill from the caliper to the lever, so that there's no trap for the bubbles to get stuck in. Give the lever quick, vigorous flicks. Most of the time, this'll be enough to get the air back up in to the reservoirs in the levers (air in there is normal).
And tell them to stop putting their bikes upside down (the Shimano manual specifically mentions this).