I am sorry to learn about your father’s condition. It is smart of him to take care of his own planning; but that is exactly what it is, planning for himself.
Your father is still the power of attorney for your grandmother and will continue to be until he resigns, becomes incompetent, dies, or is removed by your grandmother. If any of these things happen, then the successor named in your grandmother’s power of attorney will become her agent.
Your grandmother should execute a new power of attorney naming someone other than your father and a successor after that person.