After the initial burst of fitness enthusiasm is over, a client can sometimes get flaky. Suddenly training sessions don't take the precedence they once did, eating gets sloppy, and body fat and circumference measurements no longer bring good tidings.
When you recognize that this trend has begun with a client, it is your job as her trainer to remind her why she began in the first place. Revisit her goals. Mentally bring her back to that place she was when she was willing, perhaps desperate to, make her health a priority.
Your client believed that training and eating right were vital to her well being. Remind her of that in a positive way every time you can. Don't shame her. Just bring her back to where she was when she made her commitment and motivation was high. That's why she hired you.