You could be thin and unhealthy eating like that. I think it would be a good idea to make it a health attack rather than a fat attack.
Fill out a typical day at calorie count.com print out the results and show her the grade. They grade the food and show % of daily needs etc.
Maybe once she sees some numbers it could make sense to her.
Another good thing to talk about is money. EVERYONE wants to have that to spare and eating healthy can mean eating cheaper when you buy pre-packaged things.
Also the kid is at an age where he can start to learn to cook for himself. Get making healthy pizza and other kid friendly recipes with him while you are there.
Dont call it healthy with him. Just call it food. Health turns so many people off. Get them caught on the side effects of good health and it tends to stick better.