I don't get the whole 'invite the whole class' thing. Growing up I never did that. I had a few friends from school (not even in the same class), friends from soccer, kids I played with growing up because our parents knew each other through fire dept, bunco, work parties, community events. My thought is if the only option you have is to rely on inviting the whole class or just family, then maybe the parent and kid need to be branched out socially. Do parents not even do weekend/afternoon playtime with different kids?
Are you a parent? How it works is, before kids start school, their friends consist of neighborhood kids, and their parents' friends' kids. Then they go to school, and make friends on their own, not just friends of convenience. However, it generally takes a couple of years for real friendships to grow (and tons of play dates). This is why it's popular to invite everyone in kindergarten/first grade. Plus, it's the only way to get invites handed about in school ( we don't even have a directory). Around second grade, kids have their own circle of friends, parents know each other, and there is no need for invitations to go into the backpack.
Most folks RSVP here. I'm very surprised that the mom, being concerned about having no RSVP's, didn't follow up with the parents. I also think it's horrible that those parents didn't RSVP.