My 8 year old Step Daughter thinks she is fat. What should I do?

How can I get my step daughter to realize she isn't fat. She thinks dieting is normal. Her mother is constantly dieting. I have heard my step daughter's mother complain about always being fat. Now my 8 year old step daughter is on a diet, she is not over weight Her mother is letting her be on this diet.. She thinks that being on a diet is normal. Also, her mother doesn't feed her the good for you foods, she gets mac and cheese, boxed kids cuisine and things easy to cook. What can I do?