I know why she stayed. That punch wasn't the first. I'm positive of that. It starts off easy - easy for someone to excuse. As time goes on, the abuse worsens but the apologies deepen. You believe he really doesn't want to hurt you. You believe that you need to stay because you understand them.
Then you set deadlines in your head. Deadlines come and go. When you think it's better, then bam it happens again. The abuse becomes normal in some ways.
What needs to happen usually is falling out of love. When you fall out of love, you see situations clearer and more objective. Then just make that jump and go!!! When you do, you might look back and think how is that possible I stayed.