Hello @ally3 welcome to the head and neck cancer group.
You will find many posts within this group dealing with mouth and tongue pain. In addition, others may jump in here with advice.
Me, I am old school with warm salt water rinses and perhaps Orajell or similar over the counter ointments.
Tongue pain in particular is brutal as the tongue is so very sensitive to every little thing in the mouth.
Best of luck keeping your family member at ease through all this as it progresses. Maybe keep some cold water and ice chips handy as a quick pain relief which although very temporary does add some sunshine to the lamentable days ahead.
Is she getting radiotherapy from someone with experience in oral cancers? It’s important.