Some women are unbelievable

So back story and I know its not my business but I used to be friends with this one woman and we became inseparable, and a few months pass and she met this guy and they became official right away. And little by little my gut would just turning when she would talk to me about him, or if I was on the phone with her and i would hear his voice in the background and come to find out that he was starting to get physically abusive with her. I told her sisters so that they could try do something about it she didnt like that when i was just looking out for my girlfriend and she ended our friendship and he hit her one time in front of me and i went off and i tried hitting him back but she held me back because she didnt want him to hit me and he took off, she already 2 kids from a previous relationship and she ended up having 2 more kids with him. So I was so hurt about with the situation because all I did was looking out for her

So I come to find out on social media that not only does she live 3 buildings down from my mother's apartment building and yes she knows my mom and my family, homeboy is living with her so obviously theyre back together. And I know its not my business but what do I do if I were to run in to them at my mother's? I guess it sucks because what I saw her go through just to go back to him, im not saying that hes not hitting her again but you never know with a guy like that. But God forbid that happens again, END VENT