It has to do with the type of O2 masks fitted. If they are of the "quick don" type then there is no requirement for the remaining pilot to be on O2 in your scenario.
I remember something like one pilot on O2 if the other one leaves the cockpit above 300 or 320; memory fails me at the time. I don't remember anything about FL250 other than the max altitude for an airplane without O2 masks for pax.
EDIT: I found something on the Colombian regulations (which are very similar, almost identical, to the FARs)
RAC 4.18.9.2. Cabina Presurizada (Pressurized Cabin)
D) A alturas superiores a 250 uno de los pilotos debe usar máscara de oxígeno
asegurada y sellada suministrando oxígeno todo el tiempo a no ser que la aeronave esté provista de máscaras de tipo «instalación rápida» que pueda ser colocada y asegurada con una sola mano en cinco segundos y que suministre oxígeno según demanda.
"At altitudes greater than FL250, one of the pilots must wear the oxygen mask and it must be providing oxygen at all times unless the aircraft is equipped with quick donning masks"
But I still recall one of my friends who flies a 767 saying that he still needed to wear the mask at FL3XX if the Captain left the cockpit.