The advice to avoid soy is a little outdated. Just to elaborate on the above post, soy protein powder is concentrated and probably should be avoided. Soy condiments are fine but have sodium or trans fats. As for soy foods, they are fine and even of benefit. Also from Dana Farber:

Soy foods
Current research supports including soy foods in the diet of cancer survivors and does not suggest harmful effects, even for those experiencing estrogen-receptor positive breast cancer. In fact, research in patients with breast cancer patients suggests possible benefit to overall survival with consuming moderate amounts of soy foods, or 1-2 servings per day. One serving of soy is equivalent to ½ cup of edamame, 1 cup of soy milk or ¼ cup of tofu. The bottom line is that soy foods like edamame, tofu and unsweetened soy milk can safely be included as an alternative protein or dairy source, even for those going through cancer treatment.

In other words, don't work too hard to reduce soy foods!