I'm not a nurse yet (start school in fall) but my current work is with abuse victims, their family/friends, and sometimes batterers themselves. First of all, it's totally normal to feel angry! That's such a natural reaction and it says a lot about your level of empathy for others, which is a great quality for a nurse. The key is translating those feelings into appropriate actions.
Like the other poster said, it's perfectly okay to turn it over to other agencies like CPS, APS, police. In the meantime, if they're both there, try to put space between them if possible---having the abuser in the room with the victim will make it much more difficult for the victim to tell you anything or give any kind of statement. Maybe send the abuser out of the room to fill out paperwork, or down the hall to get some water for the pt, or say that it's protocol to examine the patient in private, whatever you are allowed to do in that situation. This is also an opportunity to give the patient resources...like the number for the local abuse agency/shelter/hotline.
Just my two cents
...again, not a nurse yet, but I do have some expertise with abuse issues. We always encourage people to focus your energy on the victim/survivor's needs and getting them the support they need, regardless of what's happenening with the perpetrator---hopefully they'll get what they're due in court (or through karma/judgement day/afterlife etc, depending on your beliefs!)