I'd ask for this to be moved to Relationships, you'll get great advice and support.
Most importantly be there for her. I'm not too long out of a marriage like this and the bastard effectively isolated me, so I had no feedback but his. Eventually I knew it was "all my fault".
Warn her, if you can, that this will damage her child. My H behaved exactly as you describe and the effect on my children ( not his, he has none) keeps me awake at night.
You're going to have to plaster that smile on around him for now. Fgs don't let him detect you know anything negative about him. If he sees you're a threat he'll stop her having anything to do with her.
Right now, not married, no children together she can walk (run) away from him. He might try to change this to increase his control of her.
I would think there is almost definitely more abuse going on, but this that you know is enough.
Good luck. I wish I'd had someone there for me.