What is the necessary action to take to protect the money of an intellectually unstable orphan, whose affairs my father was entrusted with by his will, especially since my father is very old and his memory has begun to decline, and there is a fear that the orphan's mother will squander his money if she takes charge of managing it?

The person you mentioned who suffers from mental instability is not qualified to manage his own money. His guardian is his father, then his appointed executor, then the ruler. However, your father's memory decline renders him unfit for guardianship, so the matter transitions to the ruler to take charge of this person's affairs or appoint a representative for him. Your duty is to bring the matter before the Sharia judge. If a person is appointed for guardianship, your father must hand over all his possessions to him.