Initial studies on soy and breast cancer strongly suggested that women with breast cancer (and men with prostate cancer) should not consume soy. However, more recent studies offer just the opposite indication. In fact, some studies indicate a synergistic effect between soy and tamoxifen. It appears safe to contain soy foods in moderation even after a breast cancer diagnosis.
Mark Messina, who has spent his career studying the topic, published a review of the literature about a year ago. Here is a link.
http://jn.nutrition.org/content/131/11/3095S.short