I'm going to sort of side with you, OP.
Name changed for this, too identifying.
I split with my ex after 10 years some time ago now. When we became a couple, she was introduced to all my friends and we tended to socialise in two larger groups. In one of the groups were five couples, all in our 30s. Unexpectedly, after we had all known each other about 5 years, one of the husbands dropped dead. Leaving behind two very young children. The now widow was a far better friend of mine than my ex, having known her much longer.
When he died, aside from family, I was the only person she contacted and wanted to see. She wanted me to speak at the funeral, which of course I did. She leaned on me quite heavily. She later organised another event in his memory to raise money for a charity which she asked me to help organise. I had had depression previously, as she now had, so we used to talk a lot. My ex had no problem.
Four years later, my ex and I split. That Christmas my friend invited my ex to spend Christmas with her. My ex has family who ever year we had to have stay at ours for Xmas. I don't have family. So I spent my first Xmas on my own and saw no one on Xmas Day or Boxing Day. But my friend, whom I had known for longer and who I helped through everything, invited my ex for Xmas.
Yes, our split was relatively amicable, no cheating or anything. I had no problem with friends staying friends. But that was a real kick in the teeth. I'm still friends with that friend, but I see her much much less and it is certainly not the same. I've never said anything, but I think she knows.