To me it is quite believiable she is natural.
Like Block says, try a steady regime of cardio 2 hours a day, mega strict low carb eating for month and months. Anyone [besides totally fat ppl of course] will get very lean. Most people have never done 2 hours cardio a day for one week, let alone weeks on end coupled with a strict low carb diet....one will melt when one does that. Try it for 2 weeks and see how much you lose...seriously.
I have never been anywhere near that lean, but get into reasonable shape. And I have crap metabolism, am not all that strict on diet, and forget the 2 hrs cardio a day, that's not happening. Having known my physique response, and many females I have worked with - I do believe it is possible to get that lean naturally with tremendous amounts of daily dedication.