Get some Vetricyn wound spray at your feed store and spray twice a day. Cleaning the wound with Hibiclens or chlorhexidene, and applying plain triple antibiotic/neosporin ointment is another way to treat. Separate her in a dog crate with food and water in view of the other chickens, so she remains part of the flock. Keep flies away or use the ointment. Give her some high protein treats of chopped egg, canned tuna, or liver. The other chick will need to be protected as well, so she is not attacked until they are similar in size.