I don't know what makes for good studying. I've never been much of a studier when it comes to textbooks. Here's the advice that I can give you to tell her:
1. Listen in class. Take notes if the class is very information loaded.
2. If the class requires extra studying, rewrite what she thinks to be the most important information from her notes and textbook.
3. Study with a partner if they can keep their focus.
The problem with her testing is most likely a mixture of two things: Lack of knowledge of what is needed for the test(s) and a lack of confidence. What she should do is start studying a small bit every night a week before a test. Whenever test time comes, she should have more confidence than usual because she will know she has studied hard and learned the material well. Over time, she will be more confident with testing. I only gather this because as a musician, the difference between when I play well and when I play badly are those two things(In MOST cases). I don't know my music well enough and/or a lack of confidence.
Of course, the ideas I have listed are something that no teen really wants to hear.(I'm assuming she's a teenager) but they are things that sometimes have to be done. I have used all three ideas to great success, and I know I'm not the only one. The biggest thing is getting her to do those things and for her to carry them out properly. Also, if she only chooses to do one thing, stress the idea of studying around 20 minutes every night a week before the test. That will allow the information to be learned better more efficiently and longer. Best of luck to her.