Tractor supply or a feed store sales spray that is supposed to help wounds heal. The folks at TSC would probably be able to help you figure out what to use.
You can flush the wound with saline or betadine. Apply some Vetericyn or plain neosporin. Vetericyn comes in a spray, since the area looks fairly large that may be easier to apply.
Keep her warm and separated. You can add some vitamins/electrolytes to her water if you wish, make sure she stays hydrated. Offer her her normal poultry food, scrambles/hard boiled egg and/or tuna.
Chickens heal fairly well barring any infection, so keep an eye out for signs of infection - pus, odors, etc.