Sounds like she is not getting all the suplements she needs just by eating well. I would suggest a suplement such as ensure as well.
Here is some information for you I found on the internet. Besides illnesses often associated with aging, poor eating habits could also lead to weight loss. According to the American Dietetic Association, changes that take place during aging result in changes in nutrient needs, and that many older adults are at risk for malnutrition. "Hunger, poverty, and anorexia nervosa ... frequently precede malnutrition," writes the ADA in a prepared statement.
Maybe she should be drinking ensure (nutrient drink) found at the pharmacy.. Or ask your pharmacist for their brand of drink. My step father was eating and drinking like normal and kept on losing weight. He ended up drinking Ensure which is full of nutrients and his weight stabilized.