This might sound sappy or cliche, but one of the best ways to help her would be to be there for her during everything. Help her study, offer to give her small 'home quizzes' after she studies (you can use her books, most of the ones I've read had small quizzes after each chapter or section). Help her study, don't let her go crazy studying though (from personal experience, even sitting still reading can be physically abusive.. don't let her sit still for hours).

Also, like Copyright said, you can try to calm her nerves for the exam. Exams really aren't that bad, and you can fly through them if you aren't nervous. It's being nervous that fails some people. Tell her not to rush (most people try to ruch through the questions and don't read some parts, which is a VERY big mistake.. some questions must be read fully and carefully to be answered, and she may come in contact with some trick questions).

Other than that, make sure she goes to sleep the night before and doesn't stay up nervously studying or something. And make sure to tell her Goodluck from AO and we hope she gets her seat.