My neighbours have a newborn baby girl.
They are about 30. Them an speaks to his partner like she was a dog. It's been going on for months.
Calls her all the f-bitches and c-s under the sun. She's so quiet and shy. We can hear him shouting abuse at her. In the garden, with the windows/patio doors open. He doesn't seem to know or care that we can hear.
Last night I heard a noise at about 8pm. Looked out of the window, to see her sobbing next to her car, holding the baby carrier. She looked totally terrified to go into the house. She started walking up the street, really sobbing. I opened the front door to go after her to check she was ok. Next thing, their front door flew open, and he half-shouted "GET BACK HERE NOW YPU FUCKING B---!" She just stopped but didn't walk back. Then he sees me, so I stepped back in my house, but still held the door open. Then he says "You just fucking wait now!!" and goes over to her. She followed him back in. I went back inside. DH wanted me to call the police. But there was but noise, just a few loud voices.
About an hour later I heard a noise outside again. Looked out of the window and saw her fastening the baby into the back of the car, and another car was there, with the engine running. I went out and knocked on the window of neighbours car. She opened the door a bit and I asked if she was ok. She burst into tears and grabbed my hand 
I said that o could hear what happened and that he sounded like he was so mean to her (understatement). She cried and said he was so horrible, and that she just wants to be a family. Told her that her and her baby didn't deserve it. She just said again that she wants to be a family. I asked her if her mum and dad knew what he was like. She said no, and that her friend had told her to tell them. I agreed. (Her friend was the person in the other car, she'd come to take her back to her house, for the night but both in their own cars IYSWIM). I told her if she came back, she could come around to our house anytime.
I can't stop worrying. DH wants to tell her parents. We kind of know them, on a 'hello' basis, and they live round the corner.
He is vile and if he's not hitting her now, it sounds like he will.
Any advice? :(