Sometimes when a client comes in with a bad Highlighting job, I can see what color I need to add to help it. Sometimes it's just tiny light highlights and some lowlights (original color)
However, if it is really bad and off color, usually the best thing to do is to cover it with her natural color, condition and start again.
Don't forget the condition part! <img src="/images/graemlins/smile.gif" alt="" />