First, I really like Karin’s answer. There is a reason your client talks so much, and that is because she is comfortable with you. As long as your client is performing the exercises correctly and safely, then I would lend an ear and let her talk (sometimes I feel like we are part trainers and part therapists in some ways!).
Trying to make her breathless to stop her talking would not be the best strategy, but you could always mention to her that you’d like her to focus on the exercise/breath/ form for a minute. That may at least get her to pay attention to the task at hand. Bottom line is that clients seek trainers for many different reasons, and your connection with the client is just as important as the workout program itself.
Best to you and enjoy!