الجواب وباللہ التوفیق

If your mother is suffering so badly that she is unable to fast or if she fasts, her health is sure to deteriorate, and there is no chance of improvement, and the qualified doctors recommend her not to fast, she is then allowed to abstain from fasting, but she needs to compensate for each fast that she could not do.

Compensation (fidya) for one missed fast per the weight of wheat is 1kilogram and 574 grams and 644 milligrams, rounded up to (1.575kg), or convert this weight of wheat into cash money and distribute to the poor.  

وأفاد القهستاني عن الكرماني أن المريض إذا تحقق اليأس من الصحة أي صحة يقدر معها على الصوم فعليه الفدية لكل يوم (حاشیۃ الطحطاوی: ۱؍۶۸۸، شامی: ۲ ؍۴۲۷)فقط واللہ اعلم بالصواب