Fairydust75, I'm so sorry that you had that experience and that the female in that house is being battered. You did the "right" thing by calling the police. Typically, the police are required to file a criminal complaint of domestic violence whether the victim wants to press charges, or not. Unfortunately, most victims of DV&A are terrified to press charges, and they're stuck in "Stockholm Syndrome."
There is nothing that you can literally "do" to save this woman from her abuser. There isn't. If the man threatened to harm you, you can (and, I would, myself) file for a restraining order against him - he's dangerous, he's violent, and it's possible that he will follow through with any threat that he makes. Your safety and well-being comes first, and foremost.
Is this woman a former friend of yours? Does she have family in the area? Does she have gainful employment? All of these things are factored into what she believes her options are. She might actually believe that she has no other options, which is typical among DV&A victims.
There are several websites that can direct you on how to alert authorities and agencies about what you've witnessed, but it is imperative that you report only what you have witnessed with your own eyes and ears. Heresay (what she told you) goes nowhere, in most cases, unless there is corroborating reports from a variety of witnesses that doesn't deviate from the facts.
The first website is for the National Domestic Violence and Abuse Hotline - this is THE place to find out what you can (and, cannot) do to protect yourself and, possibly, do to help the victim:
These next websites discuss various psychological pathologies (including perpetrators of DV&A), how to spot them, and how to avoid them:
What is important to accept (even though you may not "like" this) is that there is absolutely NOTHING you can do to make this woman "get it" that she's being abused and in danger. If she's a friend of yours, the only thing you can do is to listen and let her come to her own conclusions using affirming language like, "Yes, having my head shoved into a toilet would be very upsetting to me, too," instead of, "And, why won't you just leave?"