This sounds like a very abusive situation. He's only been physical towards her once???Aye-yi-yi once is TOO MANY times!!!!!!!!!!
He sounds like a classic abusive alcoholic. Two separate serious problems that are like kerosene & fire together.
Have you tried naming it as such with your daughter. "This is called Abuse." Just as serious as you can be - maybe back it up with some literature? It is such a slow slide for so many women into this position - they lose themselves and their sense of what is a normal relationship...
If he is angry and aggressive towards the son maybe she can get a temporary guardianship (if she wants that) and take him with her when she goes...
I'd try to get her in touch with a lawyer, just to see what her real options are for herself, and for however she wishes to handle her stepson...
Local women's groups or abused women's hotlines may have referals...
Wow - so hard. Unfortunately most of us here have learned the hard and painful way that we cannot make another human being DO anything, even if it's in their best interests.
You should be sure to take care of yourself too- reach out for help and counseling for YOU!! (((hugs)))
peace-
B.