Exact same thing happened here. A neighbor's dog did it. The dog killed the whole flock except her. She was severely injured and had no flock. Both situations hit her hard--physically and mentally. I am not a veterinarian! Here is my experience. Do NOT leave it uncovered. Do not use gauze! The new skin will quickly mesh with it and you will rip new skin off. No need to feed antibiotic if she is eating & drinking well & it seems like it is healing. With clean hands and a helper to hold her and move (dirty) feathers for you, cover it with antibiotic ointment. The bone is right there! Pull as much skin together to close it as you can, but on the back there is not much fat so usually skin is basically barely covering bone just barely. Use sterile cotton balls pulled flat on that layer of ointment. Cover this all with masking tape (not bandage tape which is hard to change), taped all over to her dry feathers around the edges. You will have to lift her wings & move good feathers aside to tape everything down all over her back. Change the cotton & ointment every three days at first. It may look like she is doing ok but she will get depressed unless she has a buddy to see eating close nearby all the time. Ours lost her will to eat much while alone. After a week she was almost dead, then we got a hen buddy just in time. She felt obligated to put the new hen in her place & pecked her, but was too weak to fight. She ate, & healed super fast after that. Keep the edge bandage on the outer dry feathers by carefully cutting off the middle wound part to change it, then re-taping onto the old edge masking tape. Over time she will remove the masking tape herself if you do not, but it is easier than gummy tough bandage tape.