It may not be healthy, but again, she might not be doing it intentionally either. When I started college I lived on the 3rd story and of course it was a large campus so I was walking a TON every single day. I also had to eat the cafeteria food as I was on a meal plan, and the cafeteria had to follow strict dietary guidelines (it was a state college) and portion sizes.
I was so busy all the time that I barely had time to eat and I was on the move none stop. I didn't do it intentionally but from the super increased activity (running half a mile to make a class to the 15 trips up and down the 3 flights of stairs) and from the lack of time to be stuffing my face (I ate three meals a day and generally a snack or two but they were small things in small amounts, a muffin at breakfast, a pre-wrapped sandwhich at lunch, a apple or cookie for a snack, etc) that I ended up dropping at an absurd rate. In 2 weeks I had lost 12lbs. Again, not intentional and probably not super healthy, but I wasn't depriving my body of food. Just really, really, reallly busy.
My advice, keep an eye on her. Praise her for her accomplishments and continue being a role model for healthy weight loss. If she continues the rapid drop in a month or two, intervene.

