Horrible to read about this .
To give some insight as someone who has gone through this, and share what it was like so to better understand the nature of this as it is not simple.
In my case its slightly different as it was family based but the same mentality applies. Feelings a victim has for the abuser are very complex and do not seem rational to the outsider.
Firstly you don't understand that you are being abused .That sounds mad right ?
Between the incidents everything can be fine , you said yourself that they seem the perfect couple, then something will happen an outburst a violent event.
what do you do ? You blame yourself, it's something you did to trigger the attack, you blame yourself, as the abuser will blame you for making them do the thing they did, the level of control this person has on you is difficult to understand to the outsider. Plus this is done by people who you believe love you, and you love them or have at some point loved them and you can have fun happy times together. To have someone like that beat you your confidence and self worth are at zero and it's difficult to be strong with no self worth. Also it just becomes pattern what happens the mind is so powerful that it displaces what happened, you don't consider yourself abused.
Secondly why don't they leave ? Well your biggest fear is fear of death.
And there is fear that you will be killed, "i'll fucking kill you , tell anyone i'll fucking kill you," whilst having a knife held to your throat so tight that you can hear the blade pressing against your skin will soon sober you, and have you acting in a ways that make no sense, it's where logic goes out the window.
"well if they don't want help you can't help them" ,
Imagining the above circumstances.
can you see why the victim might not want appear to want help?
Can you see why even the victim would defend the abuser?
Can you see why she would beg for you not to tell anybody ?
Unless the abuser is locked up for the rest of there lives from the point of calling 999 (which we know is not going to happen) the victim is fearing for there life and having been through it myself that fear is overpowering.
The chances of being killed after you leave are at the highest and most dangerous. I bet you she will have attempted to reach out or leave and that's when the violence ramps up as the abuser has nothing to lose.
Were will you go ? Without a support network the person will find it very very hard to leave as much of that is to do with is protection for fear of reprisals and not just a roof over your head.
I left only when the violence was so bad that i welcomed death , i wanted them to do it, i no longer cared i would rather be dead than put up with it, so the fear was gone, but fuck me that is dire set of affairs to go through, i was lucky it did not go over the edge, but this women, if she is being knocked out she is on the edge,
Lastly your friend, is the lowest form of scum.He really is. Shocking isn't it? that he appears perfectly normal and probably very likeable , only adding to the fact people would not believe your story police etc
The idea that he just lashes out as a sign of frustration after a drink or a rough day is inaccurate, the disturbing fact is that the people who do it, enjoy it .
There is a pleasure in what they do , I've been hit by people who are lashing out and then there's the other kind who enjoy it, it is absolute mind fuck that someone who is supposed to love you takes pleasure in what they do and i guarantee this scumbag does.
My advice is this is only going to get worse in silence. Support this women tell her family, tell the police everything you know, do not kick the shit out of him , violence begets violence, dismiss this man from your life.
She will and can only leave if she feels like she has support and protection the more she has the better chance in my opinion.
Hope the post helps people understand the warped state that a victim can get into.