She can be vegan if that's her philosophy, but I'd recommend you try to unobtrusively make sure she us getting adequate nutrients. She is growing a lot right now and needs enough calories, as well as protein and fats, if anything, stunted growth should be a good motivator to start eating better. It is so easy to miss out on essential nutrients to keep one's mind mentally alert and functioning normally on a vegetarian and vegan diet, from personal experience. Good luck.
F 28/5'4/100 lbs
"I'm not a psychopath, I'm a high-functioning sociopath; do your research."