There are loads of different reasons why people don't leave violent relationships:- fear of being alone, fear of retribution, financial worries, how it will look to others and many more. The upshot is that all you can do is be supportive until they finally decide to leave. There are lots of agencies for victims of domestic abuse these days......Well, there should be, 1 in 4 women and 1 in 6 men are abused within the home.
I am one of the lucky people who had a supportive family and friends. It was a darned good job really because the police didn't take domestics seriously back then. Fortunately, they do now, and so do social services if there are children in the household.
Be there for your friends Kitty but don't get involved in arguments with their other halves. Make sure that your friends have the information to help them get out, telephone numbers of Domestic Abuse agencies etc. They may not want to use this information just yet, but eventually, they will..... and they'll thank you for it.

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Never put off until tomorrow what you can avoid altogether.
The views expressed here are my own and not necessarily those of my family, friends, employer, this site, my neighbours, hairdresser, dentist, GP, next door's dog or anyone else who knows me..