For us tutors, we're used to having students come and go. The best thing is to just tell her that you're going to give the student a chance to go on his/her own for a while, because the tutor has done such a great job. If you haven't already written some great feedback on your tutor's feedback page, that would be a nice parting gift!
You don't have to do anything to cancel, just tell her not to come anymore. That's all you have to do!
Glad you had a great experience with her.