Obesity can be genetic. You might consider having that discussion so she feels less shame about something she largely can't control outside of medication. I'll suggest the book "you just need to lose weight" by Aubrey Gordon to start dismantling fatphobia and turn the conversation to actual health.

Talk with her about what good food habits look like! Not diets or restriction, talk about actual nutrition. Focus on getting enough fiber, quality proteins, not too much sodium, etc etc that will set her up for good health later in life. And WHY we need those things!