This sensation is most likely mucositis, and is directly caused by the radiation treatment. It will heal / resolve on its own after treatment is over, but it is extremely uncomfortable. Doctors can provide her with a topical anesthetic like lidocaine in a solution that can be swished around the mouth to numb the sore areas. This provides temporary relief. Mine was so extreme, that pain medication was necessary as well. Rinsing and gargling with sodium bicarbonate and salt water help mucositis heal faster, but does not help with the discomfort. This is one problem where all that can be done is to try to mask the discomfort, while the body does the repair work. If she has actual sores on her tongue or elsewhere in the mouth, this mucositis can become infected if not kept very clean. Try to be on top of this so that this additional complication does not add to her burden.