Others with similar situations.
https://www.backyardchickens.com/t/464496/chicken-bit-by-dog-graphic-pixs
https://www.backyardchickens.com/t/...bites-i-have-baytril-but-not-sure-what-to-use
https://www.backyardchickens.com/t/...-please-help-is-my-turkeys-wound-healing-well
Read through these threads for what they did or was advised to do to save their chickens with similar case as yours.
Saline solution is the best thing for your case. Peroxide can be used for a 1st time treatment for infection, but large open wounds, like this is not good. Peroxide damages new tender growth of flesh. After cleansing daily for many days and after each wash, apply ointments to keep the open flesh from drying out, or open to bacteria (which is everywhere).
There are several ointments that people use. BlueKote, Triple antibiotic cream (make sure no pain relief variety), and in my case I used both Triple antibiotic cream and Veterycin(horse product for wounds). You may also use honey and put some salt mixed with it, but DO NOT use processed. It must be RAW honey.
You must keep your chicken away from the others and where no flies are present. The flies will be drawn to the wound and lay eggs in it. Then you'll have maggots=flystrike.
You need to put her on a heavy does of antibiotics. Penicillin injections is best. Oral has way less value.
https://www.backyardchickens.com/t/352701/how-do-i-administer-a-penicillin-shot-to-a-chicken
Saline solution, make your own:
8 ounces of distilled water
1/2 teaspoon of baking soda
1 teaspoon of salt
Bring water to just boiling, add soda and salt, dissolves in 1min, will look cloudy.
Cool and poor in a sterilized container. If you have a glass container, boil it ahead of time and lid too.
Do not dip anything into this and contaminate it.
Pour on chicken wound.
Should keep for weeks.
Store room temp.