Could the pilot work for something like FedEx, NetJets, etc.? If he/she doesn't work for a commercial airline, his/her employer might be paying for a FC ticket and if he/she isn't flying any planes within X hours, it could be perfectly fine to drink alcohol. AFAIK the rule for most USA commercial airlines is no drinking alcohol when in uniform (and in public, in that it's hard to see how a rule on this could be enforced within the pilot's own home or when alone in an hotel room, assuming shades are closed, etc.), but there could be other pilots for which this is not necessarily a problem.