I've been a vegan for several years and was a vegetarian before that since I was 10 (just celebrated my 17th "veggieversary" actually

). You're absolutely right about it being easy to under-eat a bit since plant foods are less calorie dense than animal foods as a rule. It will likely take a few months for her body to adjust to the bigger portion sizes she will have to eat to meet daily nutritional requirements. If she looses a little weight at first, don't be alarmed as long as she's getting those portion sizes up over time so she can maintain proper nutrition/caloric intake and a healthy weight. She may also have a little digestive upset as she adjusts to the higher fiber content of a plant based diet, but that is temporary. You may also want to talk to the doctor about supplementing B-12, as it is difficult (though not impossible) to obtain strictly from plant sources (plant based sources include nutritional yeast and some fermented foods, though it is easiest to just supplement). Everything else (including protein, iron and calcium, which seem to always be big concerns) are easily obtained on a balanced plant based diet. What kind of foods is she eating in a typical day? Is she eating a lot of processed soy foods (such as the faux meats and store bought soy milk)? Those are not really the "best" to be eating, especially for a teenage girl. Home made tofu and soy milk are about the only soy "substitutions" that are really good to eat on a regular basis. The store bought stuff is highly processed, full of additives and a lot of people (especially women) have issues with it in excess. Another VERY important thing vegans have to keep in mind- you don't have room in your diet or stomach for a lot of processed or junk food. Luckily, it is relatively easy to make a more nutritious vegan version of most junk foods (like cookies and ice cream and fries and chips ext...) that you can totally binge out on guilt-free

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Is she tracking her daily nutritional intake right now? Apps like cronometer are very helpful in making sure you eat enough and identifying if there might be a particular nutrient you want to incorporate more foods that contain it into your diet. While there is a wide variety of ready made vegan foods these days, the basics really are still the best- fruit, veg, seeds, nuts, beans and grains. And you can pretty much make a whole food vegan version of anything you want. She will need to eat larger portions of plant based foods as they are not very calorie dense. For example, she could be something like a "quarter pounder" chickpea burger with avocado, tomato, lettuce, onion and a smoothie and she'd only be getting around 500 calories whereas a regular burger that size and soda would be probably pushing a 1000 calories. Fresh juices and smoothies (preferably smoothies since they also contain the fiber), a handful of nuts or a few pieces of fruit as snacks are good calorie fillers as snacks throughout the day along with hearty meals made of whole foods. And if she's still struggling to meet her daily caloric intake needs, just have her add a bowl of beans and rice to every meal. Cheap, nutritious, adds calories and a good source of protein (which a lot of people are very concerned about).
P.S. When I first refused to eat meat, my mom told me if I was old enough to pick what I wanted to eat I was old enough to make it and taught me how to cook for myself. It was actually extremely useful to learn and get used to cooking my meals so young, as I never went through that "phase" as a young adult of living off ramen and microwave meals because it was already habit to cook an actual meal with nutritional value for myself (even if it was just beans and rice with some salad lol

). Perhaps this could be a good learning experience for your daughter if she is not already familiar with cooking.
P.P.S. If there is anything in particular she likes and is having problems giving up, I probably have a vegan recipe for it. Just let me know!