I'm afraid in most cases it CAN be said to be black and white. Don't present yourself fit for duty when you clearly aren't.
Somebody who is having a hypo, feels absolutely rotten and as such should know they aren't fit to fly. I've seen people in this exact situation and there was no way they would have been capable, they may not have known the reason for feeling so unwell, but that surely is not the point. Call in sick and get to a doctor.
As flight crew we have a duty of care to our passengers, their safety is more important to me than anything else and that means if I think a fellow crew member is unfit for duty (be it alchohol induced or whatever) then I will speak out.
Of course we must offer support to people who find themselves in this situation, counselling etc, but at the end of the day it is a serious situation and demands a serious response.
If any Flight Ops director takes a line that someone is to be punished for doing their job just because of the old boy's network, then that person should be hounded out of the flying world.
Answer this, if you saw an obviously drunk driver getting into a car would you call the police? How about if it was at school arrival or finishing time and you knew they were driving past your kid's school, would you do something then? If so, why is flying any different. Professional courtesy can only extend so far.