There is nothing you can do unless you physically SEE the abuse happen. If you do, then you can call the police and they will take it from there. In some states, even if the person being abused won't file charges the state will go ahead and seek a conviction. As far as talking any sense into your friend, don't bother. Extremely rare is the abused who will listen.

I've counseled on a women's help line: Every time you see her or talk with her, let her know she's much better & of more value than how her significant other treats her. Women who have been abused have such low self-esteem & often feel very 'stuck' in their situation, without any hope. They're believing a lie about themselves. They usually believe a lie about their significant other, too, that "THIS time, he'll change for good, & treat me better!" They really do believe that, because after the abuse, the guy will 'bring home the flowers' & go through the 'honeymoon phase,' so to speak, treating her how she'd love to always be treated. The woman will usually believe, sickly think, he'll change. But he won't change, because its a co-dependent relationship. If she leaves him, then, & usually only then, is there even a chance that the abuser will begin to face himself & his abusive behaviours. But she should not come back until he's made significant progress in counseling, etc. There IS something you can do: you can set the example to your friend of how a guy ought to treat a woman! With kindness & respect, which is who you are, from what I've read of you, jay jay. All the best to you, here.

I don't think that there is much that you can do about it.