You’ve received good commentary so far and I’ll try to go over new ground.
1) Is this the only discussion you’ve had regarding her progress and satisfaction? Wiser trainers than me taught me that feedback and renewal discussions shouldn’t wait until the end of the package. As we celebrate successes, we talk about future goals and I get a sense without being pushy whether my assistance is part of that future-casting.
2) You might send a very short thank you and in it ask, “Would you be interested in providing a short written testimonial about what you liked about my training style so future clients can learn about me?”