I did that to my mom when I was 12. My family assumed it would be a phase but I'm now 34 and haven't turned back, I'm now a vegan
I agree with the others that support is the best thing you can do. If she's going lacto/ovo, she really shouldn't have a huge nutritional challenge. Unless a vegetarian eats nothing but junk, there shouldn't have an isssue. Dairy provides B-12, protein, etc. If you are really worried, maybe suggest that she get her iron checked in a few months just in case she doesn't absorb non-heme iron well. I've never had an issue but iron absorption can really vary from person to person.
If she decides to go vegan, the nutrition is a little harder to deal with. In that case I'd recommend any education you can find.