Hi there
3 weeks into radiation it is quite normal for issues to arise that you will find both scary and at times overwhelming.The rheumatoid arthritis is an added problem that will make her life a little more difficult as well.
At this stage of treatment you must firstly make sure that she is getting plenty of fluids and plenty of nutrition.The radiation will probably have burnt her mouth and she will likely have sores and ulcers which can be treated with mouthwash or if neccessary antibiotics,she may be experiencing difficulty with swallowing and diminished salive,and stringy mucous.This can be helped by gargling with salt and soda.Tiredness can be overwhelming during treatment so make sure she gets plenty of rest.
Lastly don't panic just read as much information as you can and be there when she needs you.

good luck

liz