You don't have to be a horse to be a horse doctor. It is always wise to start out by seeing a medical doctor to rule out any organic causes of the weight loss. If there is any question about the possibility of an eating disorder, I would suggest speaking with her about it and suggesting that you are concerned and want her to understand that if there is a problem, if dealt with effectively, it can be cured. Perhaps it is just from stress or active choice that she has lost the weight. The proof of the pudding will come in seeing if she can stop losing the weight after all the excitement has died down.
Osteoporosis is a common problem later in life when there has been malnutrition.
The brain and heart have been known to reduce in size when malnourishment is protracted. Many look fit and of normal weight. This is not true. Moderate, balanced eating is healthy eating. Be careful if you are a heart patient and limiting your fat intake for your health.