Do you have room in the coop for a wire dog crate with secured food and water, where she could be placed for a few days? The rooster could be pinned up for a week or so instead. If it is fluid, it should be absorbed on it’s own.
Is the swollen area puffy like it could be air under the skin? Or does it feel like swollen tissue or fluid. She might have had a peck to the back of her head that caused a leaking air sac, or swelling, or bloody fluid under the skin. I am just guessing. Do you have a rooster who might have...