There should never be a situation where one pilot gets left in there with the capability to prevent the other returning.
Maybe that's how it outta be but this is nothing new, the Captain was locked out of the cockpit on the Ethiopian 702 pilot hijack to Geneva last year:
The high-altitude drama began when the chief pilot of ET702 left the cockpit of the Boeing 767-300 to use the toilet, Mr. Deillon said. Mr. Tegegn then locked the cockpit door and took control of the airliner, he said.
At about 4 a.m., the Italy-bound Ethiopian Airlines jet contacted air-traffic controllers with a request to land in Geneva. Shortly afterward, another message, this time using the number 7500—the aviation code for a hijacking—was transmitted over the aircraft's transponder.
Authorities: Co-Pilot Took Control of Ethiopian Airlines Plane, Wanted Asylum - WSJ