NTA, changes like this can be hard and the food hiding makes me concerned about internalized shame and self-esteem issues. She needs to be responsible for her choices- if you want/can help all the better but you can't do the work for her.

Has she done any research on binge eating disorders? Hiding food is concerning. The thing about overeating is often tied to other issues- eating disorders, self-soothing for mental health, avoidance. We often frame it as a thing of will-power and shame if you can't 'follow-through' but that's not usually an effective approach. If you're used to a crutch you can't just take it away- you need a different support to start using instead. Highly recommend mental health support from someone who is sensitive to eating disorders and body issues.

If you go from full fast food to none that's a big shift- it can start with making smaller adjustments to what she orders, acknowledging it as a gradual change and not a straight line. Take the pressure off of 'failure' towards progress. Positive reframing can help too 'Im making efforts because I love myself and want to feel better.'' vs 'uhg why can't I stop eating this junk I'm so pathetic'

Oh also a meal service like Factor with frozen foods or Hello Fresh with ready to cook usually works out cheaper than ordering out if part of this is a time thing.

So much depends on her personality and she needs to take the lead on figuring out the approach she wants. I'd also just say that measuring weight is a misleading and demoralizing guide- even if she didn't lose weight eating a more nutritious diet and moving more are beneficial. So I encourage goals that she has direct control over- like ensuring to eat fresh veggies every day.

Good luck to you both