Alot of girls have issues with self confidence and insecurities about their body image and the way they look.
It's perfectly normal for a girl to feel as if she is fat and try and lose weight. Usually it's just a phase, but if it's prolonged and she starts getting boney, and i mean you can see her ribcage and the skin is just stretched over the bones, then you know she has a problem.
Otherwise id leave her to it. Fitness programmes are hard to stick to. If you want to help even more, supporting and talking to her or even joining in on her gym activity would be helpful.
If anyone wants immediate advice don\'t hestitate to contact to me on AIM (AOL instant messenger) - aim: vodkasmodka
Eccentric! Not egocentric!