You need to separate her IMMEDIATELY. If I were you I'd get rid of the rooster, unless you just introduced them recently (if they were newly introduced, the behavior's not uncommon). I have no advice for the injury except to separate the hen in a warm, dry, dark place with plenty of food and water, and to apply neosporin (non-medicated) to the wound every day. Chickens are amazing healers on their own. Good luck.