If you don't need to study, why can't you just tell her you don't need to study? If you explain yourself well enough, she can't be mad at you. After all, she asked if she could. She didn't demand, so you automatically have an out. If she needs help with the upcoming exam, maybe she should go get tutoring from someone at the school instead of attempting to impose on your. Also, since you say you're busy, you won't have time to study with her even if you wanted to. I just think you're kind of being a chicken about it.