That should have been reported by the doctor. Lubitz (the pilot) was denied an American license for this before - and somehow it wasn't caught/discovered when he got the Lufthansa/Germanwings job. Or nobody has followed up on it.

On the day of the crash he was not supposed to be on the plane at all - a paper from the doctors was found at his place after the crash declaring him unfit for duty. He kept it from his employer and it wasn't reported by the doctors neither (they didn't have the duty to do so), so the airline had no idea. Making a few of the holes in the cheese align nicely.

Pilots have the obligation to report when they are unfit for duty already, (no matter what the reason, being treated for a psychiatric problem certainly applies, though).

What was/is missing is the obligation of doctors to report such important issue to the employer when the crewman is unfit. It could be argued that it would be an invasion of privacy but there are precedents for this - e.g. failed medicals are routinely being reported to the authorities (not just for pilots - also for car drivers, gun holders, etc. where the corresponding licenses are then suspended), as are discoveries of e.g. child abuse.