I think the way society frames having body fat as being some kind of personality flaw, or something disgusting is the issue, and the striving for thinness is an issue. Weight loss jabs which just trash your appetite are an issue.
Fat is part of body composition, and a having a certain percentage range of body fat is healthy. Too much fat is not healthful, too little fat is not healthful. I see fitness influencers who are clearly underweight, and muscle mass cannot be maintained in a chronic calorie deficit, especially when fat reserves are gone. Muscle mass is one of the best contributors to longevity and health.
I suggest your daughter starts lifting at the gym, to grow muscle. This is taking lifts to failure, for minimal sets (2-3) and rep ranges around 6-10 (heaviest she can manage) and 12-15 (a bit less weight than the first), look up “progressive overload”. If she falls in love with lifting and strength training the rest will fall into place. A high protein diet no less than 100-120g a day, keeps you satiated and changes the palate. If she eats a lot of sweet treats this will be the hardest part to beat, sugar is addictive, I know it, but the craving does go away and she will be able to take it or leave it once it passes. Fat loss is entirely dependent on a calorie deficit, for sustainability I’d say 1600-1800kcal a day (with lifting at the gym minimum 3x a week), she will see fat loss, just do it every single day. Crash diets just lead to binging. Lifting heavy also builds and maintains bone density, which is particularly important after 30 years old, they will always be good habits to keep. Building muscle forever changes muscle at a cellular level (increase in nuclei) which is where muscle memory comes from - she will always regain muscle faster in the future if she has to take a break from lifting. Muscle mass is what builds metabolism, and yo-yo diets and chronic under eating and without strength training (and aging) is what contributes to the struggle to shift excess fat later in life, so eating well to fuel her body and muscle growth, particularly as an optimal body fat range is reached, is really important.
The human body is designed to run on carbs and our brains love sugar, we’re designed for feast or famine and we live in a world with surplus food, her body is doing exactly what it’s designed to do by storing fat, but she’s not in a wild environment anymore, so she needs to manage it. A food scale and accurate tracking like MyFitnessPal is helpful and not disordered as some people like to say, there’s so many foods that are surprisingly dense that it’s easy to eat several portions without realising it, like butter etc. it’s also important to track her protein as it’s also easy to underestimate chicken portions for example and she may need to eat more of some foods. Carbs are not the enemy in this story either, they’re great for our brain and pump up muscle to look fuller as they draw water into the tissue, but being mindful of calories as some carbs are just not as nutritionally beneficial as others.
Switching up the perspective that the gym is somewhere to experiment, to train, to develop and grow is a much healthier outlook in a world that seems to tell us we need to shrink.
Consistency is the cornerstone of all this, results are seen months to years down the line, and showing up week after week no matter if she’s feeling it or not is what makes it happen.
I hope this helps, as it’s something I wish I knew earlier in life.
To add.. rest is really important, muscle growth depends on the stimulus (lifting), nutrition, and rest - spread working out muscle groups throughout the week, and lifting no more than 4-5 days a week, as the body needs to recover. None of this needs to be over done, train hard, eat well, rest. I wish her all the best.