Heidi, keep in mind, what you are seeing in the mouth and on the lips is most likely happening from mouth to rectum. The purpose of the thick mucus is to protect the damaged areas and flush out the destroyed tissue. I know I keep saying this, but it is imperative to keep hydrated so the body can do it's job. Not knowing the specifics and not being there, I can't tell how much water or other hydration is being done, but it doesn't sound sufficient.
Is your mother taking guafenesin in any form? It's a must to keep the mucus as thin as possible and still flowing. There is an alcohol-free, sugar-free liquid that shouldn't burn. It's called Siltussin SA and is not long acting so it needs to be taken every 4 hours. The OTC versions have a lot of alcohol and will burn beyond belief. Ask the doctor for a prescription.
There are some other options like GlutaSolv that will help promote regeneration of epithelial cells, also.
It may take a while. It's too variable to tell but without food and hydration, it will keep getting worse.