My capacity for empathy last until the first sign of stupidity.
It's easy to say this, but if you haven't been through an abusive relationship, it can be hard to realise that they are not stupid, and they do need empathy. I am not here to say that she is not responsible for allowing that kind of verbal abuse into the household of her parents, but rather that her emotional needs, which are strongly linked to her husband right now, aren't 'stupid' -but they are confused, and unhealthy.
And a parent, even of an adult, doesn't want to upset their child more. "Don't call the police, please don't call the police!" is such a desperate plea, "He's sweet to me, he understands me..." Poor girl, but how do you as a mother or father summon up the strength to say, "No, I have to, for your own good." Sometimes you just can't. And if she'll defend him against the police anyway, it might just be a waste of time.
A way of understanding her feelings might be if you consider it this way:
Let's say -100 is total abject misery and suffering, 0 is neutral, and 100 is pure joy
Some couple might spend a lot of their lives around 25, and then he does something nice: an expensive dinner or gift out of the blue- and she's at 40 for a little while. Her happiness lifts him up too, and this kind of building produces a more or less happy relationship. Of course it dips when he goes out to have a pint when he promised to come home early, but it goes up after the apology and a day of forgiveness.
But another woman might be at -35: she's unhappy with and at herself, and on top of that, is told or shown her unimportance. Then she gets verbally, maybe physically abused. -50. -60. She goes to her family: 'Get away from him, you deserve better, you're a good person...' Whatever gets through bumps it to, what, -55? -50? Then he sobers up. He comes home. He's crying. He loves her. He can't believe what he's done, "look, I saw this bracelet and I knew I had to give it to you." And -50 is now -10, maybe 0, maybe 5!
She's constantly 'objectively' worse off, she never hits near the first couple, who aren't especially high, but look: the girl in the first goes from 25 to 40: 15 'emotional points'; but the abused/dependent girl goes from -50 to 0: she gains 50 points. She only got 5, 10, from her family, because she hasn't got any way of believing them (it would require her to see herself as a valuable person deserving of hearing those things). The differential from him
feels better (like how $5 is a lot if have $0 and nothing if you have $1 000 000) so she goes back, she gives back, she loves and is loved -or,
believes she loves and is loved. That emotional feeling combined with that belief is so powerful she can't leave. Even if she did she would probably seek out the kind of person who gave her that kind of 'high'; not knowing that a consistent level of contentment is actually a lot 'better'.