My Boyfriend just found out his child isnt his after eight years of her knowing.?

My partners ex wife just told him his daughter isnt his. His daughter is eight years old. The thing is the ex has known this scince the child was eight months old and continued to keep this a secret from him. The father was his cousin and the best man at their wedding. After finding out the child wasnt his she then had a baby with him, divorced him shortly afterwards and then took him for child support on BOTH kids for the remaining years. She has not paid back any money owed to him and now has gone to his cousin for child support. She has now come out with the truth after all these years and we still have both children weekly but when she comes over to collect she acts like she has done nothing wrong and everything is fine and dandy. It has killed him inside but he is too soft to go off at her as he is not that sort of person. It frustrates me cos I just want to kill her for what she did to him and now she just has this stupid smug look on her face. How do i contain myself around her?