Protocol for teens

I am doing weight loss coaching for female attorneys in my city. One of my prospective clients is concerned about her daughter’s weight gain- is maybe 30 pounds overweight. The teen is a twin whose sister is not overweight. They both love sweets, fast food, etc. as most teens do. If I coach the family, how do I present a no flour, no sugar protocol to the teens? They have their licenses, so can go buy whatever they want (candy, pizza, etc). I know most fast food places offer salads, but you know they are going to gravitate towards a sandwich or burger, especially when they are with their friends. Was there a way you presented things to Christian when you changed his diet, so that he had buy-in? What would get through to a teen’s brain? Thanks for your help.