A healthy vegan diet, with lots of fruits and vegetables, is not very calorie-dense. That means you may actually need to eat more food to get the same number of calories you were eating on a meat diet. If your daughter is eating a lot of grains, vegetables, and beans (which is good) she can increase her calories by eating more, or by adding some fat (oil, avocado, nuts).
It's important to remember that some fat is necessary for healthy cells, skin, and hair, and your daughter should make sure to include some healthy fats.
Using an online calorie/nutrient calculator can help your daughter make sure she is getting the calories and vitamins she needs. Try searching for "vitamin intake online calculator." If she's not eating enough, or if she continues to lose weight, definitely talk to a doctor or nutritionist.