PBK,
Your wife's nutrition needs should be tailored to her symptoms. If she has a good appetite and is not losing any weight then she can focus on a well-balanced vegetarian diet being sure to include adequate protein sources. Soy has been an area of controversy in breast cancer but mostly in ER+ breast cancer. It is now acceptable to include 2-3 servings of soy foods in the diet for ER+ breast cancer and since your wife has ER- breast cancer you can safely keep that in the diet. Of course, beans, peas, legumes and certain grains, like quinoa, will also be a great source of protein. Additionally, I recommend that she consume whole grains and a wide variety of fruits and vegetables. In regards to sugar, excessive sugar in the diet is not recommended. However, there is no reason (from the information you've shared) she can't enjoy a treat once in a while. Please see the article at the following link
http://www.caring4cancer.com/go/canc...connection.htm.
If your wife is having trouble with her appetite then my recommendations would be completely different. Let me know if she is having trouble eating and I will provide additional information.
Best wishes to you both.