Although he'd attended at first, Merlin wasn't really a partygoer and the sudden crush of kids back at school was making him feel a little claustrophobic besides.
He'd snuck out of the party to get some air, and it was while he'd been loitering in the freezing cold that he saw someone fall from the sky, or so it seemed to him. Looking up he saw the broom, put two and two together pretty quickly and then drew his wand, running over to the form and freaking out about what kind of mess he'd see once he got close... apparently, no kind of mess.
He could swear that he'd seen the kid fall though. "Uhm... are you okay?" He asked cautiously. He couldn't see any horrendously broken bones.