He has nowhere to run. Getting in the airlock would only slow him down, because helmets are a completely necessary thing, and he needs something between her and him, preferably something with a lock--

The bathroom door.

He knocks down a wall of the pillow fort, hoping to slow her down, and sprints into the bathroom. The door slams shut and he locks it hurriedly.

...Great. Now he's trapped, and she's outside with the powdered sugar.