I can see why you're hurt by this, OP. You are bound to be extra sensitive about it, what with DS not being invited to parties via school and with his upcoming ASD testing. For what it's worth, I do think it's poor form to not have whole class parties in primary, at least up until later years, and I do think it's poor form of friend to invite DS then rescind on the invite.
That said, have you considered that your friend may be in a difficult position? Are your boys as close as they used to be? Could it be that her DS has specifically requested to have just his few closest friends from school there, or that he is concerned that his school friends might not get on with your DS in such a small group?
It can be very difficult when you want to do what's best for your child on their birthday (which is, like it or not, what HE wants for his party) and also keep the peace with friends when the expectations of the two don't marry. Her son shouldn't be forced to have anyone at his party he doesn't want there, even if his reasoning is shallow (eg, if he's worried that his school friends may be unkind to your DS and wants to prevent that).
Of course, it may actually be that she have booked a 6 maximum activity and messed up the numbers? I can see then why it would be easier to drop your DS than a child in her DS's class at school while others in the class are still allowed to go.