I agree.
Your son has confided in you which is a great first step and you need to be careful you don’t push him away.
I personally would tell him that her behaviour is unacceptable and it’s only going to get worse.
Tell him that you should report this to the police but you won’t IF he tells her to move out and ends the relationship.
Tell him he needs to tell her that her behaviour is inappropriate and that they can’t continue to live together like this, so she needs to find somewhere else to stay.
You can continue looking after the dogs if this is her excuse.
I would tell him to give her a couple of weeks to find somewhere else.
In that time she is possibly going to ramp up her abuse, either in a nasty way or a manipulative way.
You and DH then need to sneak around and listen out for anything abusive she says to him and try and record it.
I hope that there are not many times when you’re both out of the house. If there are I’d consider getting a voice recorder or camera with sound and position it in a place that just happens to be near their room.
Many abusive women will turn it around and play the victim, so having recordings may help your sons case in the long run and you’ll have something to have control over her.
I would try and get some evidence if you can, without either of them knowing.
But if you hear her physically hit him or say something awful like she’s going to kill him or that she’s going to claim he’s abusing her etc you storm into the room straight away and tell her to get away from him and that she is not welcome in your house (if it’s been recorded then even better).
Tell them that you are ringing the police.
When they ask you not to, say you will not ring the police on the condition that she leaves that day and has absolutely no contact with each other again.
Do not leave them alone together from that moment on and tell her to ring the council to say she’s homeless and they’ll put her up into a hotel.
Your son can stay downstairs whilst she packs her things and tell her if she needs to contact about the dogs then she contacts you only.
If he tries to leave with her then you threaten to phone the police and say you’ve got the recording so he can’t even deny it.
It sounds like I’m dragging it out but unfortunately the police won’t actually do much, especially if he denies it which he will.
He’s also very likely to leave with her or continue the relationship because he feels bad for her, as PPs have said.
Having that carrot/stick of you threatening to phone the police may be a way to get her out of his house without him going with her, especially if you have proof in the recording.