What makes this an especially challenging situation is that because of the cognitive impairment, this individual is likely no longer able to rationalize.  Given this inability, she cannot appropriately discuss the issue with you, nor can she process or understand anything you may say that is contrary to what she believes to be true.  First, if you have not done so already, it is highly recommended that this issue be brought up to this individual's physician for a full assessment.  If possible, if you could find out if this individual has a history (prior to the onset of the disease) of disordered eating; this would likely be helpful information for the physician to have.  As I am sure you understand, it is essential that this individual maintains a healthful diet and receives proper nutrition.  It is important for you to continue to gently and calmly encourage eating and any kind of caloric intake, even if it is something like nutritional supplement drinks.  Also, perhaps by taking the direct focus away from getting this individual to eat, you might more easily coax her into taking in food.  For example, make mealtime a social activity by eating with her, offering small amounts of food (finger foods may be a good bet), all while talking, playing music, or even watching the TV-anything to diffuse the focus and to distract her away from her concerns.