I'm Vegan for health reasons, I'm currently going through the menopause.
I've been Vegan in the past, as a "clean" eating plan was suggested by my Consultants and I decided it was just as easy to be Vegan than the faff of adapting a animal eating diet. My Hindu Vegan consultants were very supportive about this. The science, as well as ethics is easily found to support a Vegan lifestyle choice.
The supplement that you linked to isn't what she needs.
Teen girls can benefit from extra Iron and Vegans have to do research around complex Proteins.
Most teen girls diets aren't nutritionally high enough for their needs.
I look to Malaysian /Thai/Indian/Creole etc cooking.
I do eat Lentil and Chick Peas a lot, both are complex Proteins.
There will be a local Vegan Facebook page, as there are Vegan athletes, as well, giving dietary advice.
Mike Tyson has turned Vegan and he has blogs, as will the Vegans that she will come across on the FB page.
There is a growing body of Vegan Sports people.
You will find links to the Vegan magazines, one newly launched which is sold in WHSmiths.
There are lists on the internet of all products that are Vegan, it's easy to be Vegan and Kettle crisps etc are sold in discount shops, so it isn't expensive and the cost of say Vegan chocolate is evened out by the lack of expensive animal products.
Aldi is very good at food labelling and most of their products on "special" days have a good Vegan selection.
I know I'm biased, but i would be very proud of any teen who cares enough about our world and it's people/poverty, to make such a personal change.