I suspect that while the person may have transgressed company policy if, in fact, he had alcohol in his system, I'm not sure he would be in breach of the Act.
My reasoning (please to correct) is that he wasn't performing an aviation function within the meaning of the act, at the time he was tested, and by the time he was to be called upon to perform an aviation function, he could be below the limit.
A strict reading of the definition of aviation function:
(1) For the purposes of this Part the following (and only the following) are aviation functions-
(a) acting as a pilot of an aircraft during flight,
(b) acting as flight navigator of an aircraft during flight,
(c) acting as flight engineer of an aircraft during flight,
(d) acting as flight radio-telephony operator of an aircraft during flight,
(e) acting as a member of the cabin crew of an aircraft during flight,
(f) attending the flight deck of an aircraft during flight to give or supervise training, to administer a test, to observe a period of practice or to monitor or record the gaining of experience,
Was the person concerned required to do anything except sit down, strap in and shut up until required for relief?