It's bad to throw up on another person.
Emilio is riding in the back seat of the car next to his friend and ate something earlier that is not agreeing with him.
Emilio wants to feel better.
Emilio informs the driver that he cannot hold down his meal from earlier and the driver pulls over so Emilio can barf outside the car door.
Emilio doesn't let anybody know how his stomach feels and tries to avoid barfing until it is too late, and he barfs all over his friends.
Emilio's friends are grossed out but grateful for the warning beforehand.
Emilio's friend is completely grossed out by the situation and stays made at Emilio for quite some time.