Is my grandmother not held accountable because of her limited understanding and her confusion between prayer times, or should we continue to guide her?

If your grandmother is aware of the actions of prayer and comprehends matters at certain times, then she must perform the obligatory prayers. However, if her permanent state is one of lack of comprehension, then the religious obligation is lifted from her, and you are not required to try and convince her. For the one who is obligated, prayer does not cease to be a duty as long as they are of sound mind, and they pray according to their capacity.