The most appropriate guidelines to include on this subject would be absolute zero tolerance for consumption of alcohol by hotel employees while they are on the hotel premises. This is the only way to ensure that guests will have confidence in the hotel staff.
One could possibly argue that staff should be allowed to consume alcohol while eating lunch or dinner. However this is not a good idea. Guests who saw the staff drinking would surely worry that they were going to be waited on by intoxicated staff members. Even if drinking were done out of sight of guests, an employee might still smell of alcohol afterwards. This would make a terrible impression on guests.
Hotels are in the business of making guests feel safe and well cared-for. There is no way that allowing employees to drink alcohol on the hotel premises would serve this goal. Therefore, the hotel code of conduct should strictly forbid drinking of alcohol by staff on hotel premises.