Practice, practice, practice.
Come up with a short (few miles at most) loop, and keep repeating it. As she knows the course, she knows what is ahead and can work more on the actual driving than constantly worrying where she is going. Once you get the loop nailed (ie consistently no problems negotiating it) then try varying it with new bits to build up the confidence.
Let the instructor work on the manouvers, you just get her plenty of seat time