Its not just what you eat, its about the energy you expend. If calories in equals calories out, then you won’t gain or lose weight. If she eats a lot but burns all the calories with exercise or a high metabolism then she doesn’t gain weight. Same for you, if you eat healthy food but only burn a number of calories equal to what you eat, then you won’t lose weight.
Yes, some people have a higher metabolism than others (exercise also increases your metabolic rate btw). That means that some people can eat more than others and still be at a caloric deficit. But that doesn’t change the fact that if you are not at a deficit, you won’t lose weight, and if you are at a deficit, you will lose weight. Even with a slow metabolism. Eat less.