My best friend's ex hit her ON MY FACE!

Last night my best friend (the one I'm telling you on my last post) was hit by her guy (now her ex) on my face. I was sitting near them while they are talking regarding their break up and then i was shocked when he hit my best friend. I came up to him and slapped him on the face! How dare him!! My best friend did not cry nor revenged, she just told him, leave me alone and never comeback. and that's it. but the guy cried and said sorry to her. he did not leave so it's us who left him alone. and now the guy keeps coming back to the girl. and i have a new information as of the moment, they are married. a secret marriage was made. and maybe that's why my best friend keeps coming back to him. I did not know anything about their marriage because it was really a secret between the two of them and the guy's dad. I wonder what will happen to them. What can you say about a man who hits girls, what about if the man has a mental disorder due to an accident when he was a baby he fell downstairs from second floor and the doctor said that there would be a mental effect when he grew up? could that be an exception to his acts? Please let me know your thoughts.