Due to your choice of words, which do not sound like you are without judgement
old boy network with his favourite medical man, was displaying those unusable characteristics, “unusual and abnormal behaviour”, must lie about the alcohol / drug use consumption question,
, I would steer well clear of any of those dilemmas.
Who are you to decide about an MD that he/she would willfuly commit fraud? Who are you to decide that a philosophical discussion about the meaning of life is a sign of mental problems? Who are you to conclude that someone "must lie" during their physical?
If you are serious, talk to a lawyer about your (and the others') legal position if and when you should decide to act on your knowledge, and whether that knowledge will stand up in court in the first place.
I have my doubts.