Answer
i certainly haven't seen anything that supports this, but i know that in many people the test anxiety isn't really rational. So trying to fix it rationally does nothing. So i would try something different - like - have her do her tests with you always using the same type pencils, or the exact same pen. then after SEVERAL sample tests with you, she could take the practice tests pens/pencils with her to the real tests. the pens/pencils would act both as a reminder that she knew the answers, and also as a sort of magical talisman.