In reply to this debate i, sadly, have been in this situation.
My capt turnd up to the flight looking and acting abit worse for wear.
felt like something was not quite right...
i took my time, kept a close eye on him all the way to the aircraft, didn't want to jump to conclussions too soon. finally once in the cockpit i new i had to confront him. had to be on a personal level and tried to use my tact, thought as low key i could keep this the better.
Asked him if everything was alright? and if there had been any problems at home...
One thing led to another and finally he asked if i felt something was up? i told him i felt that he was un-fit to operate the flight and that he had put me in a very un-compfortable situation. i gave him 2 choices...
either he leaves or i do?
thankfully he agreed to report sudden sickness and leave the flt deck. a replacement capt was sent and the flt departed.
later on that week i contacted him and explained that it was for every ones best interests and that no more was going to be said by me....i stuck by my word and since then there has been no hard feelings between us.
hope this helps.....