I am looking in on this thread to see if there is anything else I can do to help YOU SB. I posted to say that I had walked in your shoes in 1980.
I had Best Friends Mum call me from Ireland to see if she really WAS with me. Friend wouldn't speak to her on the phone and I had to jump through some hoops because she also hadn't told the truth. She didn't want Mum to know that she had made a massive mistake in selection of a DH because parents had never approved.
I wish I could remember how we came to accept Best Friends decision ot return to the house after a month of living with us. (We also worked together and travelled to the same college in separate cars - 5 minutes apart - so nobody at work would suspect. Talk about subterfuge and covering up!)
Probably one advantage I had was DP being a police inspector. He never had to hit anyone (which I understand that you want some big burly men to do) but he was very good at giving a 'Quiet Whisper in a Shell-like Ear'. He made it clear that the abusive husband would be arrested immediately if there were any further signs of abuse or aggression. Fortunately, friend's husband knew he had the power as an inspector in the are (though not their locality) that he was not making idle threats.
So my advice, in this day and age, is to get the police domestic violence unit on your side. Report in the first instance on 101 and then start to log texts, phone calls, emails - messages through friends etc and build your catalogue.
Your friend will probably try to diminish what happened. She will find excuses because 'he wasn't always like this'.
Always remember that it is HER relationship and SHE makes the decisions. Your role is to support and ensure that she remains safe. My DP walked my friend's house before she returned and worked out which bedroom she should have (escape routes - calling for help) and put a bolt on that door. He drew up a plan for use of the kitchen so that they never met up. He moved her wardrobe etc into the chosen bedroom and then met friend's husband at the door when he came back. It was my DP who explained the house-rules for 2 weeks until friend got the buy-out mortgage.
No violence, no aggression - just very clear rules and expectations.... with consequences that WOUL happen if they are breached.
I am thinking of you and will keep looking in.