Indido, does your daughter help with the cooking and shopping (I know, not cool...but vegan cooking can be awesome, especially with the right cookbooks. Have I mentioned the cupcakes?). I would tell her that, while she's welcome to eat what she wants, if she wants more than rice and vegys, she better learn to cook, and how to seek out the good stuff, even in a less than stellar grocery. There is also a list on the web somewhere of "accidentaly vegan" foods for her to check out, should she miss some of her junk food faves (Oreos!)
There are a lot of myths concerning meat and dairy free diets ("the'yre all skinny and malnourished...you can't be a serious athlete on a vegan diet...you won't get enough protien"...) and if she is choosing this route, especially as a growing teen, she'll need to have a firm understanding of what her nutritional requirements are.
Also, if's she a web surfer, have her check out the Post Punk Vegan Kitchen (theppk.com). It's full of like minded individuals, and if she loves food, she's welcome there, regardless of where her politics stand.
Feel free to email me if you would like more info :)
Here's me...shutting up again...