Burntbrat, we had a similar "friend", except, we'd invite her to do something or go somewhere as my child is an only child. I"d set it up with the parents and at the last minute they would cancel. This included theme parks as well as just going to the beach for the day. My child was always upset because it would happen at the last minute when we couldn't get another friend to go. I finally told the parents that they were hurting my child by what they were doing and they didn't speak to me either. Which I could have cared less, but they did let the kids play together, but we didn't invite her to anything else after that. It's so hard with other's kids and their parents.
I agree with Muduh. They owed you and your son the common courtesy of calling and saying the play date was off. How horrible for your son sitting around waiting.
You are probably best just letting them play at your house.
However, I would have called the mom. But that's me and I don't reccomend everyone doing that. Because it doesn't always go well.