well, shit. i can't say i didn't expect this (i commented in your first post), but i was hoping the outcome would be different.

i'm just going to say that in my experience, which is more than it should be and not limited to merely my mother, your cousin's gf's response is not remotely uncommon. there's often a bit (read: a lot) of stockholm syndrome in abusive relationships, and you'll often find that abused women will baffingly try to protect their abusers when others get involved. they do this for myriad reasons, including but not limited to: a) shame; b) conditioning ("it's my fault, no one understands - i drive him to this, if only i changed a, b, c about myself he'd stop" because that's what they've been told over and over and over and over by their abusers); c) hope that it will change if they just give him enough time/do everything right; d) fear of retaliation in the form of worse beatings, injury to property and family members [that's one of the ways my stepfather kept my mother around - threatening to burn her house down with me in it], death. often all of these things coalesce into what you're seeing now.

my mother did this for years. defended him, went back to him, the one time someone called the cops told them that it was just a spat and to go on their way, believed his apologies and hoped he'd change, believed his threats, feared she'd get it worse later, and on top of all, she felt deeply ashamed that she was in the situation in the first place, and taking the help from shelters, groups, family members would've been tantamount to admitting she was weak and powerless enough to 'put herself' in the situation. notice in my original comment i said, 'when she was finally ready to leave'. it took her ten years to get to that point, despite multiple family members - my older sister, her husband, two different cousins, her sisters, and most of all ME - trying to encourage her to get the fuck out. no one would ever confront HIM, but family members did indeed confront HER, and she brushed it all off.

in the end, when she left, it wasn't because she finally realized she could do better, or that she was sick of the beatings. at the time she left, she still thought she deserved them. she left because i, at 14 years old, told her that if she didn't, i was going to move in with my friend and her family. that's what did it. and she had to pay him off to GTFO - using what was little left of my dead father's small life insurance policy to do so. (she recently apologized to me for doing that, and i was dumbfounded - 'are you kidding? you leaving him was worth a thousand times that life insurance policy to me.')

unfortunately, her story wasn't the only time i saw this bear out. three years after my mom threw her husband out, my sister embarked on her own abusive relationship, which she stayed in for ten-ish years. hers was even worse, and in 1992, my mother and i (by then in california), flew her out here to escape. this because he had tied her to a chair, beaten the living shit out of her while she was tied up, and held a gun to her all night. when he finally nodded off, she managed to break free and flee to a cousin's house - on foot, 15 miles away. he called my mom, my mom bought the plane ticket.

three months later we woke up to find her gone. she went back to him.

the only reason SHE left HIM seven years later was because he started fucking her teenage daughter (above the age of consent, but still a minor). and she didn't leave for what should have been the obvious reason. she left because he was cheating on her.

and my friend, who just recently left her own abusive relationship - which we all (and by 'we all', i mean every single one of her friends, both her parents, and both her sisters and their spouses, and even HIS friends) recognized as abusive within the first month of their relationship and spent FOUR YEARS trying to get her to leave. and when SHE finally did, it wasn't because she was sick of it (i just saw her last night - six months later, she STILL believes it was her fault he treated her in such a way); it was a rather banal argument over a home they were purchasing together that did it.

so. all my personal stories aside, but in light of them, the best i can do is to offer you my commiseration, and encourage you to be there for her if she ever reaches out. otherwise, it truly is out of your hands, and despite what others have said, 'calling the cops' isn't necessarily going to do anything, because when they show up and she tells them she's all right and they leave (or when they take him to jail and she opens her door to him the minute he's released), she's right back where she started. this doesn't mean you shouldn't, if your ever witness to the abuse again (you absolutely SHOULD) - it's just to say that ultimately, this comes down entirely on her. her fate is in her own hands. i can only hope she finds her breaking point sooner rather than later.