I'd say this is a great opportunity for her to learn about personal ethics. It's all fine and well to hold a value to be true, but if we really want to 'live it', it takes effort, education and personal accountability. I'd tell her that if she wants to live by this value then she needs to research and understand how to keep a healthy, balanced veg diet and know about the impact on her body if she doesn't keep up iron, protein etc.. And she'll need to learn to cook some basic veg meals as you shouldn't be expected to cook different meals for different members of the family. If she's really serious about it, she'll put in the work. If it's just a phase, she'll revert back to what's easiest - having you cook the standard family meals for her!