She will definitely benefit from practicing prior year exams that are easily found with a google search. I would not focus too much on how well she performs but just on the experience of taking the AMC test. If she does not really like math then I certainly would not force her to take it.
The AMC will cover topics like number theory, counting and probability that she probably did not learn in school but if she is interested it might help her see what else is out there. The Art of Problem Solving has many resources that will help her if she becomes interested in doing better next year.
