In essence the mental health aspect of the responsibilities of the healthcare professionals is obvious. If Lubitz had said he had plans to crash his aircraft into a mountain then yes the professional had to do something about it.
I dont think in actuality it would have been that clear, even with hindsight. The normal questions asked tend to follow a formula. Are you feeling suicidal? That is a yes or no question. This is followed if the positive is given, Do you have any plans? If its a no then whatever the suspicions of the medics then they are not on firm ground to do much.