I was in a previous relationship and lived above a couple that we were friends with, the man worked with my ex do and we'd often socialise as a 4.
That ex was horrifically abusive, nasty and bullying in so many ways so fortunately I am out of that relationship.
At his and the other man's work, there was a woman that my ex suddenly got mentionitis about. The woman I lived next door to also felt insecure about this woman.
Apparently she'd been invited to a work night out (I was never invited on them) and this other woman was trying to lead my partner away by the hand and also criticising the other couple's relationship, saying, "I hate couples who don't give each other any independence."
They'd also been on a night out and this woman invited my ex back to hers 'for a few drinks'. He didn't go at first, but he did one night then lied about it for a week.
Naturally at this point I was very concerned, as was my neighbour/friend. She'd tell me how she couldn't stand this woman.
Anyway long story short, I confronted my ex, he initially denied it, but ended up cheating with this woman and they were in a relationship basically the day after we broke up, even though I still lived in the property.
Anyway I told my friend. She was absolutely furious, called them every name under the sun and she said her partner/my ex's workmate, was also furious at how I'd been treated, and apparently didn't speak to him for weeks.
Anyway I moved out, and I had a lot of support from this couple. We stayed in touch, and I occasionally went back to meet up with the female friend, also saw the male in passing.
However gradually, my friend started to be 'busy' every time, and didn't stay in contact as much, I was confused. Anyway then I learned via social media that she had become friends with this woman. I mean their partners work together, it's not my business. I can't dictate who she's friends with, I guess she feels awkward or whatever but it's a shame she phased me out like that and felt she had to take a side. Ah well. It happens I suppose, what would you have done?