This is no different than the person that goes on a shooting spree, and after the fact neighbors and family talk about how unsettled or unstable the person was but no one wanted to speak up. You have a much better picture than someone who is just meeting him for the first time, and about to spend four days in a cockpit with him. This guys livelihood shouldn't even enter your brain, you're understandably trying to justify not wanting to act.

If everything you say is true, from an objective outsider looking in you have a moral obligation to at the very least report everything about this to the FAA and his employer. Every accident has a chain of incidents, only we never read about accidents where the chain is broken early on. The doctor enabling this also need to be held accountable. I would confront them and tell them either stop, or you'll be sending letters to not only law enforcement but their state certification boards.

After that, if this guy is serious I would get ready to defend yourself. Conceal carry classes, buy a gun (or many), etc.

You have a volatile situation, but sometimes doing what is right is the hardest thing of all.