Unfortunately, even though the proceedures are not particularly conventional, you canot repsond to a disciplinary issue with a grievence. Ie caught drinking at work on more than one occcaision cannot be the fault of the boss who gives him plenty of hours albeit too many some weeks. He'll be lucky to keep the job.
If you want to help, suggest alcohol abuse support rather than employment law. If you follow the legal route, he will lose the job as the pub owner could lose her licence for employing him drunk.