Just to throw a wrench into the Magic Mouthwash debate, I was prescribed a combination (I don't know the exact formulation) of:
benadryl, tetracycline, lelix and nystatin
They bill it as "Dr. Bob's Magic Mouthwash" and has to be put together by my local pharmacist.
Make sure that your mom is on the proper pain medication. If her throat is sore she will not want to eat and drink properly which will only lead to huge complication in the next several weeks.
Remember radiation is a "cululative" treatment - each treatment adds to the previous ones so it will definitely get worse before it gets better. Pain medication in your family's ticket through this.
My radiation oncologist and technicians prefer that you don't use any lotions, hair conditioners, mouse, hair spray, etc during the treatment period on or near the treatment area. Maybe elimination of some of those items will help keep the burn to a minimum.
Good luck.
Donna