Hi, Crystal. People with MG sometimes lose weight because they have trouble chewing and swallowing, or because they're on a drug that suppresses their appetite. But if your friend has a good appetite and is eating plenty, and still losing weight, then someone should check to see if there is another cause, because that's not a symptom of MG.
The first thing that comes to mind is Graves' disease (hyperthyroidism). This is an autoimmune disease that is very common in people who have myasthenia gravis. It's also more common in the elderly. Hyperthyroidism speeds up your metabolism so that you lose weight even though you're eating a lot.