[Paul F]

Your neighbor is taking on the serious liability of underage drinking. They will be directly liable for allowing it and any consequences of that activity. To make sure they understand this and that it is on record, have the police contact the parents and notify them that the police have the underage drinking on record and the parents have been warned. They are now on record for any future instances, with no excuse to say they didn't know. They will be liable for future outcomes. It may not work, but if they have any sense, it might sink in and cause them to take some responsibility.