O.A.
Hi there. Prenatal vitamins with calcium are the best for someone of that age - if you can't find one with calcium then give her one of the calcium chews (the chocolate or butterscotch flavored ones) with Vit D in it.
I say prenatal not because we want her to start thinking about kids but because she will get the iron and folate she needs to avoid anemia and prepare her body for adulthood. Vit D we are discovering has implications for prevention of cancers and chronic pain issues and many folks, women in particular, are depleted in this wonder vitamin. Calcium because she is building bone still and the calcium she has now will forestall osteoporosis and other bone issues later.
Hang in there with her. The human body is amazingly resilient and she will be fine!