The dreaded alccohol!
About 20 years ago, I was a FM with an LGW airline. We had a captain who was always "smelling of drink" and I had received many complaints from cabin staff. We could never get to the bottom of things until he was caught out on an overseas detachment by the FO who called me one night and told me the capt had been drinking heavily during the early evening and was now in bed, but what should he do? I explained that as he was the only other crew member there, if he still suspected the capt was under the influence in the morning, it was up to him to stop the flight even if he "went sick" himself.
In the event, the aircraft got airborne and flew the sector and I assumed all was well. The CP had been brought into the story however and grounded the capt on landing where he was breathalized and found to be well over the top.
The individual was dowgraded to F/O and the we placed him on a monthly monitoring regime by the company doctor. He agreed to this action, but within a couple of weeks requested to leave the company. Although we felt he was taking his problem away with him, we let him go. He joined another small airline and within six months had been fired for drinking on duty.
Sadly the pilot concerned died later, but my point is that one can take a horse to water..... but.... We were prepared to go an extra mile for the guy but in the end he was determined to keep drinking and there was only one outcome possible.