Chiron,
I forget your Mothers health history at the moment, but if she has low HCL then a water-only fast would indeed be an ideal solution. This will give her digestive organs a complete rest and allow them to recuperate.
The question is whether she will agree to undergo one and for what length of time? Convincing anyone of the benefits can be frustrating.
Fruit is the next best option as (as you have said) it is a predigested food and is the least taxing (energy-wise) of any foods.
Could you get her to see someone like Dr Alan Goldhamer or Dr Fuhrman perhaps? with the addition of some current fasting literature from either of them?