Troisgarcon, what you wrote from the other womans point of view is entirely reasonable. But it would still be tough shit.
If she had been saving to include her dh in a holiday, that should have automatically included his child. If that means saving for another year, so be it. It is not a case of adding an extra person on, it's a case of excluding a child from the start who should have automatically been included.
I would also guess that if that post was to be found on here, that there would be plenty of advice on how to improve the realtionship and help the dsd to have a nice holiday at the same time as everyone else. There would also be plenty of post saying that that is what you chose when you decided to marry a man who already has a child, so if you want your husband in your life, you just have to get on and deal with it.
I know from my dh's pov that it is hard to be a step parent, especially when the children have such a close relationship with their Dad, but he wanted to marry me, so he made the choice to deal with the difficulties that arise. He is a grown up who was more than capable of making that descision, as was the woman in the OP.
There are two sides to every story, but I'm not sure that the other side of this story is relevant. Simple fact is, a little girl is being rejected from a family holiday with her own Dad. How can anyone think there is any possible justification for that. There just isn't a way to justify it. There are NO circumstances where that is acceptable.