Fair enough. Sorry if I'm sticking my nose where I shouldn't. I've been the overweight/unhealthy teen before. I dieted throughout my teen years and it's not fun. The difference was that I was extremely unhealthy (as in I was at my current weight at grade 10...). The mentality of trying to fit in and be healthy can be a daily challenge- especially when the only thing people really have in common now are sitting down around a table to eat.
If it helps at all maybe a sports team with a schedule would keep her active. At that point it's not a "I don't feel like exercising" so much as a "my team is depending on me being there". It has an heir of responsibility and accountability and would help her feel like she's a part of a group of young girls.