Lubitz was able to fly in this case on the condition, that he get periodic psychiatric evaluation. The problem with this is, apparently the results of this evaluation are self reported. In order to be able to fly, there should have been a mechanism to waive doctor/patient confidentiality, where the doctor could be required to inform the airline immediately, that Libitetz was unfit to fly.
I am of the opinion, 20/20 hindsight of course, that he shouldn't have been permitted to fly at all, but it's pretty ridiculous that an illness that can affect public safety, and can remain concealed, be only self reported. Require people to sign a waiver, if they are flying on such conditions.