If she is fussy with food, maybe think about sticking to similar foods each day - meal prepping some lunches might help. I recently lost some weight and found that a monotonous and predictable diet worked best. I allow myself more treats now, but still stick to it mostly. I know that, roughly, this amount of calories will keep me on track. You don't need to share the number of calories with her, but perhaps check whether the portion sizes are correct - if she's still hungry, maybe see whether you can up her portion of meat and veg with the meal, so that it's still a filling meal, just more balanced.
I've never been into any form of exercise and, really, getting a grip on my diet is the only thing that will control my weight.
This is the kind of thing I have most weekdays:
Breakfast - protein yoghurt and an apple
Morning snack - small bag of popcorn
Lunch - grilled chicken kebab, pitta/cauliflower rice, salad - all with a low calorie, spicy salsa or tzatsiki
Afternoon snack - apple
Dinner - grilled meat (pork, steak, chicken, koftas, meatballs, bolognese etc), green veg, potatoes/pasta/rice - I vary the seasoning or sauces throughout the week
For evening snacks, I make little snacky plates to pick at while watching TV which come to about 300 cals and make me feel like I've had a treat - mini bags of haribo or skittles, wafer bars, fruit, carrots, baby cucumbers, ice lollies, popcorn. The mini bags really help me with portion control!
I also agree with getting rid of any fizzy drinks - squash is fine, but I found even sugar free pop had an awful impact on my gut. I'll have a can of cold san pellegrino once a week now, as an actual treat. Sauces like ketchup and mayo can also up the calories in a massively, if she has them.