It’s the low fat, low protein diet that does it.
Have your Mom start taking 5000 units of Vitamin D3, try to convince her to increase protein and animal fats.
The Fosamax is not a good idea. I followed the above and totally turned around my osteopenia.
She won’t listen to me. I’ve given up and am just watching my own health regime. Agree about Vit D. And Weights! (not Barbie weights, either).