Legally, that would be wrong, but personally?
I'd kill them and quite happily take the legal punishment, and don't think she'd stand by you as has been said she is probevly so dependent on him at this point than no matter what she'll try to rationalize away whatever is done to her.
I've seen women in abusive relationships before, it takes somwthing drastic to end them, often somones death, usually hers.
While I understand those that advise not doing anything, I am deeply disapointed in all of you, you claim that murder is wrong, that the human liufe is precious?
These people have given up that right, they have reduced someone elses to a life of torture, saying that it's up to her to get out, well it's not that fucking easy, most are trained to believe that they are the ones at fault, or that nothing is happening at all.
Take the legal route if yyou can, but do it soon, she'll hate you for it, she'll deny it ever happened but with councelling she may yet have a chance.
I just know that I wouldn't be able to stand by and let it happen.