This is when vegetarianism stops being healthy, as sadly a lot of swaps for meat are quite carby. I don't know if this is similar to your DD, but one of my DD's friends is veggie and seems to live off of cheese + tomato pasta and potato dishes which are atrocious for the waistline if you don't work them off.
I know you can't really control what she eats outside of the house- but encourage her to make healthy choices and make sure there is plenty of fruit to snack on. If you think she's buying chocolate from the school tuck shop, don't give her any money to take to school. My DD put on so much weight through school, because unknowing to me she was having a Mars bar both breaktimes -.-
Does she like quorn or Linda Mc Cartney veggie sausages? Is she a lenient veggie who eats fish? Make sure you have some protein in your evening meals and lots of veggies- but try and give smaller portions of chips/potatoes/rice.
Most importantly, don't call it a diet- just say it's getting healthy and do it together as a family :) you can go for walks together, cycling, swimming to get some exercise in.
Sorry if this is a little bit gung-ho, but my 18 year old DD is currently going to Weight Watchers as I didn't install the importance of healthy eating in her at a young age- and I wish I had :(