don't worry you did what you thought was right it's one of those situations were you damned if you do and your damned if you don't. If you hadn't told his parents then sure he would still be your friend but if you hadn't and something happened to him and they found out you knew they would blame you for not telling them. Live well as your consious is clear and thats the main thing as in the end we all die alone.
and gypsy he said he went to parties with him he didn't say he went to parties and drank with him. i have a few friends that go to parties and don't drink.