Sleepingbunnies I had to do the same thing for my best friend and work colleague - but it was before the days of Mumsnet. I was 'winging it'.
The police phoned at 3am to ask if I would give my friend a temporary home.
The police gave her a car escort to support her driving her car from one side of a city to another but then sort of 'passed her over' to my DP on the door step.
My DP noticed that her car was covered in blood from the previous altercation (OW beating up her DH and home) and went out with a bucket and sponge at 5am. That seemed bizarre when I was trying to settle best friend and find out what had happened.
But with hind-sight my DP and I played to our strengths. He didn't want the local bobbies knocking on our door asking about copious amounts of blood on friend's car (not least of all because he was a Police Inspector himself!) He also couldn't do the emotional support for my friend - but he was great at providing fabulous home-cooked meals. he made my friend very welcome and didn't invade our space as she worked through what she wanted to do.
Now my friend stayed with us for a month. But in that time she decided she wanted to keep the house (no children) and arranged a buy-out mortgage. She wanted to go back and take possession so my DP put a huge bolt-lock on her bedroom door. He checked other safety measures in the home and had her registered with the local police. She lived with the alcoholic partner for about 2 weeks until she bought him out and he moved his stuff.
End of? Not on your life!
They started dating again 18 months later - just doing breakfast dates in a local cafe. Was I happy? Never .... but it really was nothing to do with me. They never did get divorced and after the alcoholic husband was sober for 3 years (with the support of AA) he moved back in.
They are still together - still no children. They have a fabulous life together, although it is compromised. Friend's DH won't socialise at all where there is alcohol so friend attends weddings, parties, christenings and even BBQs on her own.
I tell you this story because I also could have cheerfully murdered my friend's DH that night. But the marital relationship was my friend's to 'manage'. I just had to be there to support, listen and help. I couldn't chastise or disapprove when she wanted to make up with him. I could express an opinion - but she always had to know that I was there for her.
They are off on a cruise for their 40th wedding anniversary next month!
So, focus on your friend and guide her. Get her to look at the Freedom Programme, but always remember that the decisions are hers.
Good luck to you, your DP and your friend. I hope she finds her right path.