My son said his mom was hit. Advice?

My son (7) came home from his mothers and didn't want to talk me about a secret. We have a custody agreement where he goes to her place on weekends. He loves her alot (or so he says) and I know he needs a mother in his life. However he said she told him not to tell anyone. She got into an argument with her fiance and he (fiance) put his hands on her. Then she told my son to get out of the car. That's all I heard and it's happened before. However she always Denys it and says hes making stuff up. I want to bring it up to her but I know it'll be denyed, and my son will get scolded for "making stories up". Hopefully someone has some ideas. I want to help him, but simply talking to her will not work. Any ideas? via /r/Parenting http://ift.tt/2jzdzFa