Can illnesses like malaise and fatigue be an expiation for abandoning prayer for many years before the illness, and what can a son do for his mother who left a sum of money for ongoing charity after her death?

The mother's abandonment of prayer while she was healthy and able to perform it is a major sin, one of the greatest sins, and may even be considered disbelief by some scholars. However, now that she has reached a state of illness that has deprived her of her intellect, prayer is no longer obligatory for her, due to the Prophet's (peace be upon him) saying: "The pen has been lifted from three," and he mentioned among them the insane. There is no expiation for the missed prayers. Rather, what benefits her most is frequent seeking of forgiveness for her and giving charity on her behalf. The money she bequeathed to a specific cause must be disbursed accordingly.