I'm a few months removed from breaking up with my 25 year old BPDexgf. It was a difficult experience. A lot of unnecessary pain in my opinion. Anyway, I was just thinking about the incredibly supportive role my friends played during the experience and how their involvement not only helped me recover but also influenced my decision-making.
To start, my friends saw that this was a bad relationship for me MUCH SOONER than I did. In fact, after my ex and I had our first big fight, I remember emailing my married best friend (her and husband who is also one of my best friends have about the best relationship I've ever witnessed two people having) and telling her everything. She comforted me and then told me in no uncertain terms: "I'm so sorry, but this relationship is not healthy for you, and the amount of joy, happiness and support you get from her will be greatly diminished by the pain." She didn't tell me to break up with her, she gave me her opinion, and of course her support. This was a theme. None of my friends judged me for staying with her (even through the recycles), and they always made themselves available to listen and provide support when crazy stuff was going on (and I needed a lot of support at times

).
After all of this, by the time I came to the end of my relationship with my ex, she had thoroughly alienated all of my friends. And as weird as it is to say, that really helped me stay away for good. Firstly, I knew my friends were probably seeing things more clearly than me when it came to the relationship, and their only dog in the fight was my happiness. Secondly, I knew it would be impossible to have a fulfilling relationship with someone who I couldn't integrate with my closest friends.
I don't know, I was just thinking about this recently and wondering what other people's experiences were like when it came to their friends. How did they help, maybe in your case they didn't, etc.