You are doing a wonderful thing for this teen, especially considering the painful memories and feelings this seems to elicit in you.

First of all, your friend's daughter lives in the same culture we do -- she knows that being overweight is a problem on many levels, especially if she is as bright and well-read as you say. She has already experienced the stares, the embarrassment, and likely the cruelty of her peers as many of us did as teens. Warnings about life as an overweight teen or young adult will not be news to her.

Since she already has an interest in loosing weight, I'd offer LC to her as an option, and offer yourself as a role model. Teens are slaves to pop-culture, so she is bombarded with messages every day that loosing weight means deprivation, and exercise is for people who are already fit and look great in their work-out gear! In my opinion, your best role here is to dispel these myths , rather than tell her what she already knows.

While it's true that with teens, things go in one ear and out the other, that is mostly the case with things they're told by parents (and sometimes teachers.) Your best asset here is that you are NOT her parent. If you approach her as the "cool friend" rather than as "admonishing adult" you'll get further with her. This is not to say you're attempting to lecture her, but much of what you say will likely be taken as criticism at first (regardless of how self-aware she seems to be) so your tone is key.

Does she have access to the internet? Show her this site. Start a jouranl with her and let everyone who has posted here give her encouragement.

Personally, I'd forget the letter, and just put yourself out there as her mentor. Be human, discuss your shortcomings as well as your successes and she'll probably open up to you in a way she can't with her (normal weight) Mom.

Good luck!