My approach is a bit different.
She knows he's abusive, which means she also knows she should leave him.
You can't force this, and nor should you - however much you want to.
You need to do your best to ensure her partner doesn't see you as a threat.
In your shoes I would do the research, find the solutions, to help her leave safely so that when she's ready you can give her the answers. It's important that she feels in control of the process because she's losing so much control in her life and it takes bravery to leave.
Every step of leaving can be scary and there can be confused loyalties, fears of what will happen if she ends things, if she reports things...She may not want him to get into trouble. Or she might want to just move on and forget.
Some things you can do...
(1) Look at escape planning advice on both refuge and women's aid.
(2) Look into the process for getting a non molestation order with powers or arrest attached (which will mean he can be arrested if he breaks it).
These can feel daunting for the victim. Some tips - she can stop the process at any time, she should request her new address & contact number aren't on the order, any time she has a worry or fear about it she should ask the legal team about it. Eg. She might be worried how he'll react when he finds out about it, but often abusers back off because they acknowledge what they've done when it's written down.
(3) come up with a codeword for her to let you know when she's in trouble (and needs 999) or is ready to escape so that she can talk to you safely around him
(4) provide a secret mobile phone that she has access to with an easy to remember pin. You could disguise this as your work phone (and won't you be so relieved when she finds it!) just in case he comes across it
(5) offer to keep safe important documents and sentimally important possessions
(6) help her come up with an escape plan
(7) reassure her that what she does is up to her, if she chooses to leave, when she chooses to leave, how she chooses to leave
Continue to support and not judge if she leaves and then goes back to him. Most victims of domestic abuse take more than one attempt to leave successfully.
It can be tricky to keep him onside, let him believe you're not a threat, while bolstering her at the same time but it is a line that it's possible to walk and if you can get the balance right then you're less likely to get cut off.
Also reassure her that even if she does stop speaking to you and years we're to go by, you'd be there for her when she needs you. Often women end up isolated having lost their friends & family through the abuse and then feel they can't reach out when they're finally ready to leave. Let her know she always can.
Some useful websites:
https://refuge.org.uk/
https://womensaid.org.uk/
https://www.ncdv.org.uk/
Good luck and don't lose hope