hard to say not seeing everything in its entirety. here, you can use force to stop such an act in defense of yourself and others. the law states you don't have the "right to kill" but you have the right to use deadly force to resist such an attack on you. or others in "theory".
i am not a LEO so can't necessarily insert myself into situations - but its a judgement call in this case. if he is cutting her and beating her, i think i would have to act to end the situation. i may wind up in court on charges or being sued by his family. its a possiblity. but could i live knowing i sat by and watched her be killed or maimed when i could have possibly prevented it?
oh yeah...if i pull my CCW, i will use it. not gonna talk or threaten or any such negotiation. the time for that is past if i draw. its a last resort. someone told me once that if your only tool is a hammer, then everything starts to look like a nail. the danger as a civilian is overreacting and lack of control. unfortunately in many cases, our over-litigious society puts us a a distinct disadvantage at the git/go.
things don't always end like we fantasize. talk to the LEO's about domestic squabbles where they both turn on the police who are trying to help. this probably ain't a domestic call, but to think the victim will be gushing praises on you after you shoot the guy may be a false hope.
all that being said, i think i may have shouted at the guy and tried to call him off or distract him - i'd definitely call 911, if i had the chance. if he was actively cutting, stabbing, i don't think he'd hear a sound from me except bang, bang, bang........