The folks I know with MS ..if they've experienced any change in weight at all, it has been an increase, not decrease. And while indeed muscles may lose strength over time, if they're not being used, that extreme a weight loss and muscle mass loss should be more carefully examined. Muscular dystrophy type illnesses can cause that, diseases that directly affect the muscles -- not MS, which directly affects the nerves.
Be sure to talk with your neuro about that when you next see her. And you might have your gp call and report his findings, just to confirm that for you.