I understand how you feel, jannah, almost like you're damned if you do and you're damned if you don't take any action. What about peace as a a social norm? It's not our call nor should we take it upon ourselves to try to measure out the level of violence to determine the seriousness of a domestic dispute if things are being thrown and they're screaming so let the police come. Every single time.
Why settle for less than peace. There are kids who are in the middle of it all and the more they experience without help arriving, the more they have to tolerate as a part of their lives. No one will feel safe.
The one time you don't call may be the one you wish you had called.
Of course the woman is going to be angry, she's got the spotlight on her and her husband. I'd congratulate her for surviving the ordeal and inform her that the police will be called again if she's fighting again. I guess the bottom line is you can only help someone that wants to be helped. She might thank you someday but don't count on it.
InshaAllah kheir, Im saying duas for these people to have peace restored. ameen.