Welcome To BYC
You mention a vet visit - did the Vet say "Newcastle" or give the respiratory infection a "name"?
Did they prescribe an antibiotic, what is it?
What's the inside of her beak look like?
What do you feed?
Does she have a bad odor about her head, face or inside the beak?
Where does she live - outside? Can you post photos of your coop/run?
Where are you located in the world?
What treatment(s) if any, have you administered so far?
Any lice or mites?
I'd follow the vet's instructions on giving her care. Work on hydration and eating.
For the feet, clean them with Chlorhexidine. Place her on clean dry bedding and keep the feet dry as possible.