Can't believe people are so appalled by underage drinking... especially considering that the guy will only be two days shy of his 18th birthday.
Anyway OP, here is an idea - you could photocopy your passport if you have one and fiddle with the numbers (including the ones in your passport number, which incorporate your date of birth, if you want to do a pro job). It will take 20mins and just requires access to a photocopier, scissors and glue. If they ask why you have a photocopy - just tell them that you didn't want to take your passport on a night out as it is too valuable.