It’s a horrible feeling when we think we no longer know who our friends are anymore, it must be a pretty hard and sad time for you right now.
However, have you considered why your friend has suddenly changed her behaviour? I can’t say this is the reason why, but I have a feeling she was a vegetarian and so forth because she felt it connected you two, so that you two had something you could bond over, and the reason she has changed could be a sign that she is just trying to be herself (some people genuinely don’t like the vegetarian/vegan lifestyle) or it could be her way of telling you that maybe something is going on with your friendship, and that she doesn’t feel as close to you anymore.
Also, on that note, have you also really examined your feelings for your friend? I couldn’t help get the feeling that you only really seemed to like her as friend when she shared your views and lifestyle, but now if she is actually expressing how she feels (that she doesn’t really like the vegetarian/environmentalist lifestyle) you seem to want to end that friendship because it clashes with your beliefs. If you’ll forgive me for saying, if your friendship was truly deep and meaningful, you wouldn’t be so quick to throw it away over something like this. Again, sorry for saying, but I feel that maybe your friendship wasn’t all that meaningful to begin with, if all you care about is her changing views.
Have you thought about talking openly, honestly and without judgement or anger with her? Also, if this behaviour is really her, have you considered maybe getting to know this side of her, to try and deepen your friendship? If that’s something you don’t want or can’t do, then I suggest you end your friendship, otherwise you’re both going to become frustrated, hurt and drift apart which won’t do both of you any good.
I hope you two stay friends, as friendships should be cherished.