Is that her vent? It looks like she may have had a prolapsed vent and that attracted severe pecking and cannibalism. Were the others pecking her? Could a predator have got into the coop? What do you have for first aid? Vetericyn wound spray or similar, Chlorhexidene 2% or Hibiclens and water, or plain saline (1 tsp per quart/liter of water) could be used to clean it. Then keep applying plain Neosporin or Triple Antibiotic Ointment to it twice a day. Keep her separated inside a dog crate or pen with food and water. How is she behaving? Will she take some water or food with water added?