M.O.
For these purposes, I'm going to keep it very simple.
Hitting is NEVER justified. Ever.
The way to prevent it to leave.
And that's what the abused wife needs to do. Leave.
If these questions are being asked, the relationship is long past saving. Your friend needs to get out, and not look back.
I recommend the website http://www.youarenotcrazy.com/.
I also recommend that your friend stay in a shelter for a bit, and that she seek counseling. It's very hard to take the first steps out of an abusive relationship, but after those first few steps, it gets easier. You can help her by looking into shelters in the area, and by giving her a safe space from which to make a call, if needed. But let her make the call herself.