I would guess she isn't moving her arms enough if she lets her arms just hang at her sides its probley affecting blood flow. I would get her a pair of cheap treking poles to try. it will elevate her arms and get the blood moving easier. Wallyworld or alot sporting goods stores will have them for around 20 bucks, I have heard the swiss brand from walmart is a a good value.
I had the same thing happen to me every once in a while. I was letting my arms hang and it was cause the blood to pool. Its the same as locking your knees when you have to stand up in a choir or wedding can cause you to faint from the blood pooling in your legs.
TThe treking poles have helped with the arm numbness. I would also think the poles might help take some of the strain off of her knees as well especially up and down hills. I've noticed they take a fair amount of pressure off my knees.