The human epidermal growth factor receptor 2 (HER2) is a gene that can play a role in the development of breast cancer. Approximately 15-20% of all breast cancers are HER2-positive, an aggressive and fast growing form of breast cancer. Current medications that target HER2 on breast tumors have improved patient outcome, but resistance to these therapies remain a major problem in treating this type of cancer. The HER2 regulates cell growth and survival. This study will evaluate a new treatment using a potent Poly polymerase (PARP) inhibitor known as Niraparib. Niraparib will be combined with trastuzumab, a HER2-targeted agent, to evaluate the safety and tolerability in patients with metastatic HER2 positive breast cancer. It is anticipated that the combination of drugs will improve survival and have few side effects.