How do i help my teenage niece lose weight without dietpills or expensive diet plans?

I'm 16, and I was never overweight, but I lost a shitload of weight from running. If she runs 2-3 miles every day for the whole summer, she'll lose quite a bit. If she doesn't run at all now, she'll have to build up to it. It depends on what kind of shape she's in. If she's really fat, she'll have to start by going around the block for a while and build from there. It'll be a while before she go a good distance. If she's in decent shape already, she should be able to run a short distance for about a week or two, and then move up to 2-3 miles per day. Seriously, if she does this, she doesn't have to change what she eats at all, as long as she takes in roughly 2000 calories per day.