Is it permissible to help a mother suffering from memory loss during her prayer, such as reminding her of the bowing (ruku'), prostration (sujud), and recitation, and is it valid for us to pray in congregation with her to help her complete the prayer correctly?

If a Muslim is unable to perform the prayer in its complete form, he should perform what he can, and what he cannot perform is waived for him. This is out of Allah's mercy and facilitation, as Allah the Almighty says: (So fear Allah as much as you are able) and the Prophet, peace and blessings be upon him, said: (When I command you to do something, do as much of it as you can). Thus, whatever duties of the prayer a servant is unable to perform are waived for him, and he must strive to perform the pillars and duties, even if by seeking help from someone who reminds him or prays aloud with him.