Oh my goodness….that’s intense.
I have unfortunately seen similar done to a female red rump parrot when her mate decided to try and kill her and basically completely scalped her.
She survived through impeccable hygiene, keeping her warm and in a quiet area strictly where flies could not access her.
She was regularly gently sprayed with HEAVILY DILUTED betadeine and a syringe of freshly made sugar water was regularly dropped onto (but not into) her beak for her to drink every few hours for a day or two until she started eating solids. I believe she actually started eating normally within a few hours but sugar water was continued to be offered for a few days- always freshly made.
surprisingly she made a quick and full recovery.
This was done by a human nurse in an extremely rural area where there wasn’t another option
Obviously vet treatment is the best course of action if at all possible.
but taking you at your word and knowing it isn’t always possible and plenty of countries/locations just do not have the option of a vet or one willing to see small birds, the above is all I can suggest