I guess it's a very hard call for the whistle blower, and one not taken lightly. I would hope there was at least some discussion of concerns between the crew, with the aim of the suspected offender to fall on one's sword and call in sick.
Not really a hard call at all when lives are concerned.
I once witnessed this situation. The captain of a five man crew turned up quite obviously the worse for wear. It was known that he had been to a very late party in quarters the night before. His crew gave him some very strong hints that he seemed to have a heavy cold and should declare himself unfit to fly, but like an idiot he tried to bluff it out and continued to prepare for the flight. At that point his crew "blew the whistle" on him to a squadron exec, who immediately had him replaced. I know he received a huge rollicking and possibly some other "unofficial" punishment, but there was never any official disciplinary action. The pilot concerned distinguished himself in the air some years later, and received a gallantry medal for his actions, so perhaps the line that was taken was the best in the circumstances, but doing nothing was never an option.