Is there any way you can take her to a vet?
Found a couple more links about treatment as said in the post above by @chicapee 
 nystatin or copper sulfate is a possible source of treatment as well.
You might want to pm @casportpony 
 to see is she has any suggestions. You will have to keep her hydrated so you may have to tube her. I'm sure it's very difficult for her to swallow so this may help. It sounds like you are practicing some safety measures in keeping her from you flock, I would keep her isolated and make sure to take precautions to avoid cross contamination.
http://www.pets4homes.co.uk/pet-advice/how-to-treat-canker-in-poultry.html
https://www.backyardchickens.com/t/1080266/canker