Interesting scenario
No self respecting medico would turn up for work while intoxicated / under the influence (remember the sex/drug-addicted Malaysian neurosurgeon case ?).
Even in first year, it has been drummed into us the key principle of 'First, do no harm' so I guess unless there is no one else, a drunk medico should not offer his/her services.
Hypothetically, were the medical skill required (say intubation) and there is no one else, a drunk medico (preferably with recent experience) would still be a better bet than an untrained sober FA unless you're already half comatose yourself from EtOH.
Liability wise, who knows the implications until it is tested in court.
The way I read it, you're covered as you're trying to help where there is no other alternative.