How drunk do you get till you vomit?

My cousin and his gf stumbled through the door, my cousin obviously drunk. He fell onto the couch and almost immediatley fell asleep. His gf left soon after that telling me to make sure he sleeps on his side. Im really young and never had to look after any drunk person before. Its 5AM and i have to get up early but i cant sleep because i feel completely obligated to watch him all night(morning?). Hes on his back and snoring like a demented bear. What to do?