You are describing me, a year ago. I am in environmental law, and I was a veg for 18 years for (primarily) enviro reasons.
I'm guessing you guys are at most 20 or so, and as I recall, when you're that age, you don't want to hear much advice from other people - you want to do your own thing! But The Vegetarian Myth + The Primal Blueprint are the two books that turned my life around. She probably won't appreciate and/or read the book you give her, but you could leave a copy lying around. I guarantee you, sometime when you're gone, she'll pick it up and have a look. I would have. And don't expect her mind to be changed right away. It may just plant a seed. It took me a year after reading Veg Myth to give up my veggie ways. Depends on how long she has been a veg, though. Leave the PB lying around too.
One thing for sure: wheat/corn/soy agriculture is by far the most destructive force on the environment. Greater than deforestation, greater than cattle ranching. Eating pastured meat is generally less environmentally destructive (if it's locally raised) than eating that wheat cracker.
One more suggestion: Give her a book called "Rewilding the West," by Richard Manning. It's all about the destruction of the Great Plains, and how enviro groups are trying to rebuild them. They want to reintroduce bison to the newly restored native grasslands. When bison graze native prairie, they actually improve conditions for endangered animals that would naturally be part of that ecosystem: black-footed ferrets, prairie dogs, foxes, migratory birds. It's an inspirational book, and really got me to thinking how if we could get Americans to prefer pastured bison meat to beef, we could support a completely restored native prairie that would no longer need to be plowed under to grow grains for ... cattle (and sometimes us). Plus of course bison meat is so much better for you than wheat crackers.
Good luck! But my advice is, don't push. Veggies like to come from a place of moral superiority (speaking for myself, at least) so trying to undermine them that way will earn you nothing but a more rigid vegetarian. Logic is what finally persuaded me - I bet the same might be true for her.