You could explain to her that the liquid diet doesn’t work. She should be able to eat the foods she likes while losing weight. She could safety lose weight by being in a calorie deficit and being more active. Doing a liquid diet would mean she would be hurting her body. :( while doing the calorie deficit she would be able to safely lose the weight and keep it off. If for some reason the liquid diet does work… it would cause her to gain it all back when she starts eating normally again.
Hopefully this helps!!