Is there any nice way to tell a person that he/she is fat and should stop eating

I think you should talk to her parents and not her. They are responsible for her at this age and are likely to be the ones that are buying her all the food that means she is putting on weight. I wouldn't talk to her about it, its a sensitive age when girls are getting more aware of their bodies and her confidence could really suffer if you say the wrong thing.

She is indeed big. You can tell her that if she doesn't stop eating like that no one will court or like her if she becomes teenager. You can also tell her as a joke. But do not overdo it. I have a niece who now has anorexia nervosa (as we thought) because she was always teased by her cousin..

well this is a hard one but what i think is that when you see her eating non healthy food or junk food just tell her "hey dont eat that type of stuff its not good for you" and that i think will change her and she will get the messege