My oral lichen planus improved after I switched to natural toothpastes without SLS and, for more severe flare-ups, using dexamethasone elixir (rinse 5 milliliters in the mouth for 2 minutes and expectorate, 4 times/day). Made a huge difference. Also, “magic mouthwash” can help with pain. Check with your pharmacist. Stress can be a huge trigger, as can GI issues.
I hope your mom can get some relief.