Red, the bit about the Xbox bus really hit me. I have a now adult grandson who was diagnosed as on the spectrum at about age 7. Like your grandson he is very bright and articulate, he has always been skilled at computer things and would have loved the xbox bus idea. He was never invited to a birthday party. I empathise with your hurt, angry feelings. As a family, we have had similar experiences and it makes you want to rage.
As others have advised, I'd say keep your counsel. The other mother has behaved badly, insensitively and unkindly and I fear nothing you can say will help that situation. Let's hope that it doesn't interfere with the friendship of the 4 boys when they're at school.
It's already been said but I agree, he needs a special outing/treat as some kind of distraction. People can be so selfish, can't they.