What are ways to reduce your risk of being harmed in public places

Your good friend emma invited you to her 18-year-old cousin matt's birthday party. emma is a straight-a student who is also on the cheerleading squad. when you get to her cousin's house, you see that there are no adults there. emma's cousin hands her a beer. he then hands you a beer and says, "here, have one. i do it all the time, and i never get caught." what would you do?