And honestly, even if it was fine last time - they have said no, so that's that. I am always astonished by how many mumsnetters feel entitled to bulldozer other people's boundaries.
There's no hint at all that they don't want to be her friend, and her friend was cautious but straightforward about telling her the news, which honestly was a kindness.
Divorce is a very real loss and people grieve over it. OP sounds like she's still at the trying to be breezy and "we're all adults here and surely we can manage this without a fight, after all we've known each other for years, and though we're not in love any more we still feel love for one another and we do have children together!" stage of developments, and thinks she and her ex wil be friends into the future.
That's not going to happen - source: reality.
I think she has not yet realised how painful the losses are going to be, and is reacting to this holiday news from that place, her pain is not about the holiday, it is about the loss of all her hopes and dreams, the life she had, the plans she made. Her happy memories of that holiday as a happy couple are actually a good reason not to go, it won't help her heal at all, it's just an attempt to hold onto past happiness.
I do feel sympathy for her, been there, done that. But I was pragmatic about other people's involvement. Friends are not our therapists and they generally don't want to be. The two women I was able to share my pain with had been through something very similar to me, as for the rest I didn't expect special treatment or for them to let me push in as a third wheel.
And if her ex is not already dating, he soon will be (almost certainly), that just doesn't seem to have entered her head. They're not friends and they will not be friends in the future and soon he will be prioritising another woman over you. It's truly difficult to get your heart to believe that, even though your head knows it is true.
I had been separated from my ex for about fiveish years when we finally organised the divorce. I had been in a new relationship for about two years (and I went on to marry him, he's wonderful) and when the decree nisi came through I just burst into tears. In no way did I want him back (at all!) but losing a marriage is losing an awful lot and the grief just flooded over me again all at once.
I have been quite blunt with the OP on this thread, because I think she needed to hear it. But I do feel sympathy for her. I just think she is directing her upset at the wrong people, and if she wants to keep their friendship will have to adjust her expectations. She's single now, her ex is in the past, it is hard but she will have a better time and feel less "devastated" if she accepts her new reality and looks to the future..