I can see you've had lots of replies but I just felt compelled to reply...
My youngest DD, was diagnosed ASD age 3, she is now 18. She has much older siblings so she was like an only child, her siblings are diagnosed so it wasn't new to me, but my youngest had much higher needs & it had a far bigger impact on her day to day.
When I read your post I could feel it for you both because there has been so many times from such a young age that my DD had friends let her down, she put on such a brave face so many times, but I knew every time her heart was broken. It's still the same for her, there's never a week goes by where there's not at least one time, I'm holding her whilst she sobs & I lose count of the times she's tells me, that some "friend" has let her down, not turned up, ignoring messages, whilst she still puts on the brave face & says it's ok, I know they are just tired/working hard/feeling low.
It sounds like you've done brilliant & your DD is loving her holiday, I know the hard work that goes into trying to compensate for the let down, we so desperately don't want them to feel sad, but the whole time we are aware, we aren't that "friend" that they were so excited about.
I want to comment on "that message" from the Mum, the non descript "bad news" is an absolute classic bullcrap excuse. The added little sympathy line, to distract from her rudeness & she needs to make you feel guilty for feeling annoyed that she wasn't in touch -she doesn't know whether you was annoyed or not, but she she knows that most people would be, so she has to cover that angle. Then she needs you to feel sadness for them too, because then her ignoring you, is definitely excusable & it's so awful you aren't going to bring it up in front of her DD who could drop her in it.
That's not to say there hasn't been some bad news, because I expect there has been some kind of unpleasant news, at some period, because she will need to be able to answer any questions, if she is put on the spot, but it definitely wasn't just at that time & that definitely wasn't the reason for her not contacting you & why her daughter didn't go away with you. She has just chosen to make that her reason because she hasn't learnt the basic etiquette of saying to someone "I'm so sorry, I know I said x would go away with you, but, she doesn't feel like she can now/I don't feel it's the right thing" She made a conscious decision to ignore you, avoid you until it was to late & give herself time to think of the best way to lie about it.
A few reasons why I say it's bull..
*We have a natural tendancy, that when there is a genuine reason however sad/traumatic (unless it's something totally huge unexpected at that precise time, like, life ending/changing accident) our brains do this thing of "oh no, I'm going to let sheila down, I must let her know" we can be experiencing something tragic, out of our hands, yet we feel a guilt about letting someone else down, we feel we have to let them know.
*When a child is super excited about something, then we receive sad news & it might fill our head & we can forget things, you can guarantee our child won't forget what they were super excited about, they won't let us forget.
*Parental instinct, we never want to make our children feel sad. We shield them from certain things in life, we approach things in a way that will have the least negative emotional impact. When there is something we can't avoid telling our child that we know will make them sad, we will try to offer/find something positive at the same time, to soften/distract/ease the sadness. She ain't gonna give her DD no double whammy of sadness.
I could go on with a massive list of why I call bull, but I'm going to stop myself.
Well done Mum on being a wonderful Mum, you're daughter sounds amazing, she has her resilience, positivity & empathy from you & well done for being a decent human being...this other Mum, keep her at length, she's lost out!