but does she eat like that all the time? It might be that generally she eats well but pigs out every now and then and knows when to stop. I certainly know with me I just don't know when to stop (or do know, but carry on regardless!). Not only this, but people don't generally eat what they want and stay slim unless they are active as well. Most of the skinny people I know are skinny because they work at it. They eat well, they don't worry as much about how much they're eating but they're also exercising.
Sorry, if I offend, but I think it's easy to sit back and say it's unfair, but at the end of the day, most of us are fat because we eat a lot more than we put out (exercise) and these people are slim because they overall consume less calories than us and do a lot more exercise (or are more active than us).
the thing is she does no exercise! :O and eats like that all the time! Amazing