In my lifetime, I've had to deal with quite a few women who had abusive husbands/boyfriends. I've told every single one of them the same thing; get the means to defend yourself and use it.
I've heard the "But I love him/he loves me" BS a million times. No, he doesn't love you if he is willing to hurt you. If you don't act to stop him, you will wind up another statistic and he will go on to abuse someone else.
I will also tell you that, if he will hit a woman, he ain't no man.
Kudos to the young mother for defending her aunt (and most probably herself and her baby) from this waste of skin. I hope she can move on past it.