Child Custody - they were never married or living together. My fiancee has a son who is 1 . His son mom is denying visitation.

She has 5 children, including his son who is 1 year old. Recently after we announced our engagement she started denying his visitation. Said she will raise child support, which she cant because its aready at 20 percent, and she said she will take his rights away, and he will never see his son again. My finacee has been cooperative with her to avoid conflict, now she is giving him problems. We don't ahve any children and we have a lot of time to care for his son, we are currently looking to hire a lawyer. He wants to file contemp of court and have her pay fees becasue she is the reason we are going to court. If he proves she is not being cooperative, can he get full custody? With her having 5 kids and he only having one, will that work to his favor? Currently we have police reports and text