RATIONALE: Estrogen can cause the growth of breast cancer cells. Hormone therapy using fulvestrant may fight breast cancer by lowering the amount of estrogen the body makes. Monoclonal antibodies, such as trastuzumab, can block tumor growth in different ways. Some block the ability of tumor cells to grow and spread. Others find tumor cells and help kill them or carry tumor-killing substances to them. It is not yet known whether giving fulvestrant together with trastuzumab is more effective than giving fulvestrant or trastuzumab alone in treating breast cancer.