It may be an idea to get a cabin crew member/dispatcher/engineer to confirm your fears. If they agree then get them off the a/c asap. If you even think about continuing the flight you should have your licencse ripped into a million pieces.
I cannot agree with you Silberfuchs, there really is no excuse for drinking before duty in the same way that there is no excuse for drink driving. Covering for that person means it may happen again and what happens if next time it isn't spotted? Not something I'd want on my conscience.
Luckily, this is such a rare occurance (despite the usual un-informed press scare-mongering) that the vast majority of us will never be put into that situation, but if you do, stick to your guns. If you don't know what to do, why not? There are all sorts of situations that we plan for in aviation that may never come up (engine failures for a start) but that doesn't mean we don't know how to deal with them, this is one of those situations. Think up a plan and stick to it!