I'd try to cool down, first. There is every possibility that after the phone call she made to your phone, hers died. It has happened to me -- phone death at a critical moment, and no time to recharge before having to be somewhere vital. Or she rushed into the house to see her sick kid and totally forgot that the phone was in the car and not with her -- after all, you left your own phone in the car, so you know that it does happen.
I agree that she should have called you en route to her home, to say they would not turn up. That was lax -- but possibly understandable if the other child was really sick. So unless you know for sure what happened, please consider cutting her some slack, and think about how you might react in the same situation, especially if you had no idea whether your child was a little sick or instead might really, really need you immediately -- and you're not home. I know most of us would say "I'd at least make a quick call" but that's easily said when we're not the one frantically driving home trying not to hit another car as we wonder how ill our child is.
So...she should have called again, yes, but she probably is pretty mortified that she didn't. If the child is OK now, it would be nice if she called to apologize and say "let's try again." But those are all "shoulds" and "it would be nice" situations. Unless you know just what happened, I'd give her the benefit of the doubt here. In fact, if she's your daughter's good friend' s mom, why not call her to ask how her child is doing? It would be the big thing to do.
And as another poster noted, why didn't you take your daughter to see the movie then and there without them? That at least would have distracted her from her disappointment for a while.