I’m in agreement with the answers which focused on awareness and alignment to strength your clients posture. I also agree with Joanne. It is difficult to give specific exercises or programs to address the issue without knowing the clients history or seeing the clients current posture.
Here is what I can tell you. I have created a program to successfully develop a healthy posture which includes relaxed and balanced upper core, (traps, shoulders, etc). The basis for my program came from two resources: “Anatomy Trains” by Tom Myers and “The New Rules of Posture, How to Sit, Stand and Move in the New World” by Mary Bond. If haven’t see/read these books, and would like to use their innovative techniques for creating a healthy posture, I highly recommend your purchase them. Each time I read them, I gain a new perspective on how to implement ‘new’ ways to train the postural and athletic movements of the the body.