Call women's aid 0808 2000 247.
Abuse doesn't have to be physical for the police to be involved - coercive control is a crime, and that is exactly what you are describing here. If they've told you otherwise then the person you spoke to was wrong. Try again, and use the words coercive control. It's been a crime since 2015, they have no excuse for being inept about it.
Realistically, leaving in secret without warning is the safest way forward. There are ways to get him out the house and theoretically to stay away from her, but he doesn't sound like he'd respect it. So she'd be on eggshells and vulnerable to him showing up, making demands, manipulating her, etc.
Help her sort out somewhere to go, and help her leave when he's out.
Women's aid can help with where to go, where she stands, and the important things to try and bring with her.
Once she's out help her to get a space on the Freedom Programme (and probably counselling too) so she won't be as vulnerable to his manipulation and getting sucked back in. It will help her build a future life.
Be aware that even when you want to leave it is tough to do. It's frightening, confusing, and she will probably be filled with guilt. Don't expect her to feel happy, jubilant, or safe in the immediate aftermath. With support, it will probably take a few months before she starts to feel like she can see a future for herself and to have confidence she did the right thing. In the interim there will be wobbles, and fear, and guilt, and grief for the life she dreamed she would have with him.
Don't give up on her if it takes her several attempts before she manages to leave permanently. It's incredibly difficult to break free. Don't tell her what to do or give her ultimatums. Be there to give her her power back over her own life.
Going on the Freedom Programme yourself will help you to understand what it's like for her and how it will have affected her mentally. Www.freedomprogramme.co.uk