 RATIONALE: The monoclonal antibody trastuzumab can locate breast cancer cells that have HER2 on their surface and either kill them or deliver tumor-killing substances to them without harming normal cells. Estrogen can stimulate the growth of breast cancer cells. Hormone therapy using exemestane may fight breast cancer by reducing the production of estrogen. Combining trastuzumab with exemestane may be an effective treatment for breast cancer with high amounts of HER2. 