You know her infinitely better than I do, so I am counting on you to tell me what her decision would be based on your experience with her and your knowledge of her wishes. This clarifies their surrogate role. Increasingly, physicians, hospitals, and nursing homes are requiring family members to have formal DPOA appointments that give them legal authority to speak for the patient. If no such advance documents exist, and the patient is mentally incapable of decision-making, then a court proceeding to appoint a legal guardianship may be necessary. This brings up the term incompetence, a legal term stating that a person can not make decisions for himself or herself.
Mental capacity is decision-specific rather than all or nothing.
A person may be able to make certain decisions rationally but not others. Or, they might not be able to manage a complex medication schedule, but they might be able to weigh the pros and cons of having an infected gangrenous leg amputated. Medical technology has revolutionized geriatric care. Artificial kidney machines for dialysis. Feedings tubes t can feed an unconscious patient.