the answers work around the question. To help maintain good cervical posture, work the mid-trap & rhomboids. big breasts force a cervical flexion that needs to be ameliorated by retraction of the shoulder girdle, and specifically depression. that will help to bring the anterior muscle groups into good position.
I have worked with a client whose specific goal was to prepare herself for breast reduction SURGERY! Although this is somewhat different from your situation and question, I agree with some of the other posts here. Also, I would recommend that your client speak with her physician to get a better understanding of her situation and to determine if there are any medically-related issues at work here.