Sorry to hear about your mom. I do understand how frustrated you are because you can’t help her more.
Toothpaste I was told to use Tom’s children, strawberry, flavored fluoride, free toothpaste. This has helped me greatly, and my mouth is not nearly as sore. One note it does not contain egg whites so it will not foam up like a lot of toothpaste will.
Spicy foods are not her friend as they will irritate the Lichen.
Mouthwash I don’t know much about as I don’t use them, but I would be extremely careful go to a health food store and maybe they have a children’s that is not as strong as the adult ones.
Keep a diary of your mom‘s mouth sores.
What foods that affect it also if she likes coffee, tea, or soda to see if that affects the lichen.
I am sure your mom is on some medication for this right now some gel or ointment that she could put on the sores in the mouth
I wish you both a very best. I know it’s not easy, but it’s important to keep a diary so when you go to the doctor, you take it with you I wish you the very best.