Sadly, I have quite a bit of experience with this. I was the sole caretaker for my elderly mother when she was dying of ALS (Lou Gehrig's Disease).
She was an incredibly healthy woman who was doing all her own yardwork, shoveling snow, and even still riding a bicycle in her 80's. But in late 2014 she started suffering from muscle weakness, and eventually in 2015 was diagnosed with ALS. She lost her appetite, and eventually the strength to move, chew and swallow her food, or even breathe on her own. She went from a healthy weight of 125 lbs all her adult life, to wasting away down to an emaciated skin-and-bones 95 lbs at the time of her death.
Your mother's case is not nearly so severe, of course, and it's very common for elderly people to suffer from low appetite, and concurrently lose too much lean mass and strength just from eating too few calories and too little protein.
As other people have said, supplementing her diet with regular Boost or Ensure shakes will really help out with keeping up her bodyweight and strength. Boost does makes a Very High Calorie Boost shake with 530 calories per 8 oz serving. Buy your mother cases of Boost or Ensure shakes and make sure she is drinking them regularly.
With my mother, she lost the strength to swallow her food, and so we had to have a gastric feeding tube surgically implanted so that I could inject the liquid nutrition directly into her stomach. That helped maintain her weight for a while. Hopefully you never have to go that route, but it is always an option.
All the usual diet advice goes out the window in such a situation. High fat, high sugar, high calorie drinks and food are her best friends right now. Make her drink the shakes, or encourage her to eat a pint of Ben and Jerry's or Haagen Dazs every night, whatever works. But you need to get her calories up, any way that works for her.
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