I completely agree about the greater failure being in remaining in a violent relationship, rather than in admitting to having chosen a poor partner. But, she doesn't see it this way, and goodness knows I've talked with her about it exhaustively. When logic becomes clouded by denial, it seems like there is absolutely no prevailing upon it. :|
I've worked with abused children before, and frequently, the parents are in denial, especially if one of the parents had been the abuser. The child will also usually tell you that "daddy/mommy is very nice, they love me and only want what is good for me. I've been a bad child". You can point out that the things that have been done to them is sick and wrong, but it does not matter to them. They know no other life, simply.
It is not the mindset you have to break, as rationality (for a normal person who'd have evaluated that violence as being worse) is not the issue. It lies with the heart. It is usually someone of strong will, with high expectations of themselves, inability to express themselves well, strong pride/dignity. And inside, they are deeply in pain/anger. The knot is in the heart?
That's why directly confronting the issue only drives the denial deeper. You cannot replace what is hard-wired in the brain by years of conditioning by simply taking it out. That leaves a gap in them, and the knowledge that they've made the wrong choice and lived with it for years. And perhaps what they fear most is that emptiness/failure vs the constant emotional turmoil which you've gotten used to.
What I've found useful is more to refocus them onto more positive things, to engage them in more areas of life. For people who've been hurt, something to do, a listening ear, and discipline (interms of knowing when to be hard and when to be soft on themselves), changes things more. Gradually, they'll come to the realisation that there're more than one ways to live life. And hopefully, they'll learn to forgive themselves, and it is only then they can walk away. Change is an evolution, not a revolution..
Hope that helps..
* If you want to force the issue, you'd have to get a court order, if the violence becomes abuse. Where I'm from, most of the cases of abuse are reported by 3rd parties e.g. teachers. The courts then evaluate and remove the child from the parents if necessary. Harsh, but safer.