That is one I would treat with Baytril ASAP. Baytril is banned for use in poultry, but it really would be the best choice for this type of infection.
If you live in the US you can get Baytril here:
https://www.ladygouldianfinch.com/product_baytril.php
https://www.jedds.com/shop/misc/
https://allbirdproducts.com/products/baytril-10
The dose when using the products above is 0.045 ml per pound of bodyweight twice a day orally.
Edited to add:
If a vet is an option, please do take her in.
If you live in the US you can get Baytril here:
https://www.ladygouldianfinch.com/product_baytril.php
https://www.jedds.com/shop/misc/
https://allbirdproducts.com/products/baytril-10
The dose when using the products above is 0.045 ml per pound of bodyweight twice a day orally.
Edited to add:
If a vet is an option, please do take her in.