I do not know, that seems to be a toughy, with a random outside factor. The piston caliper should be fine, and the pressure of your break fluid is maintaining, the only thing that seems to be reasonable is that your cylinder boot for the caliper may be damaged or got fold up in it so it isn't allowing it to properly retract. The only other factors are your master cylinder for your break fluid.
Has he had his brakes changed anytime recently?
Anything to do with brakes? caliper? brake lines?
how far down is his brake peddle go when he presses it? estimate by inches
C25 probably has the answer
No such thing should happen as "free up" as your drive a bit. The brakes need to be free, until they are engaged. Generally there is a leak or lack of pressure and you feel poor braking capabilities. However, this is the complete opposite which is and has to be related to your entire braking system.