It’s a hard line to tread grammaretto. Our son discovered that most people found it impossible when he and his wife separated. It remains a great sadness that people he thought were his friends turned their backs on him.
In a similar situation, many years ago now, the woman I thought had been a good and close friend for a number of years simply disappeared. Just didn’t respond to contact. Her husband, who we knew less well, came to tell us and brought their children, in hindsight to say goodbye. We’d known them from babies. The family had lived in a different part of the country so no accusations of taking sides, not that we would have taken notice, I mourned the loss of them all.
Thinking about your current situation do you have to align yourself with one or the other now? It would be sad to lose a friend you value. I wonder if it would be possible to support them both. I guess it depends on how you feel about doing it, definitely not easy, also how each of them feel about you having contact with the other. Its hard for sure and I can certainly understand why many people simply focus on one of the former partners.
Don’t forget to look after yourself through this process. As far as friendships go in my view it’s like dropping a very large boulder into a pond. The ripples are felt at more depth and distance than the separating couple realise.