Big Hilly,
Concerning your hypothetical scenarios, in virtually all those cases I think I'd talk to the person about it. After all, he/she is a mate. I would be concerned to try to get them to change their behaviour, which would make a lot more sense than either shopping them or turning a blind eye. If that didn't work, then I'd have to decide what to do next...but that would depend on their response to my concern...and I'm talking about concern, not confrontation.
IMHO. that sort of approach usually works best. Not that I always do it very well. When I tried to do something about a friend who was developing a serious drinking problem, she lost her temper and didn't speak to me for months. Mind you, she did stop drinking so much too. I think I've learned since then not to be too blunt...or maybe to be blunt, but gently.