All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If your grandmother is sane and capable of fasting without any consequential adverse effects on her, then she would be required to fast. The fact that she is bedridden does not necessarily mean that she is incapable of fasting. However, if she is unable to fast because of her old age and the fasting proves to be harmful or unbearably difficult for her, then she would be allowed to break her fast and feed a needy person for each day missed. This feeding should be paid from her own money. If she has no money, then feeding will not be obligatory for her either.
Allaah Knows best.