I think that if one KNOWS that someone is clearly dangerous or illegal then Yes by all means they should warn prospective passengers if given the chance.
However there are so many busy bodies in GA who would just love to become "Air Police" that it makes me shudder to think of twitching clubhouse curtains.
Regarding paperwork issues, lets say I let my JAA license and medical laps yet still carried on flying, would I be illegal?......(the answer is No by the way).
Our aircraft logbooks are now with our maintenance organistation, they take care of all the entries in them, I don't get to see them unless I ask. Now there was a case not so long ago where one maintenance organistaion was falsifying logbooks....so now what?
Just pointing out how difficult it is to dob people in.