This could be an interesting incident.
If the chap was going on as a relief pilot, and did not board as part of the designated cockpit crew, how would one determine what his B.A.C. would be when he finally went into cockpit to assume flying duties.
If he had stopped drinking more than 12 hours before he got into a flying seat, and his B.A.C. was by then below the proscribed limit would he be committing an offence, or not?