Your caterer made "extra" money by not having to serve as many people and the people who didn't show up lost out on a "free meal" (not to mention the celebration of your wedding ... or the birthday party in question).
But you -- and the mother of the birthday boy -- did not suffer a "loss". Sure you would have paid less to the caterer if you knew ahead of time ... but absent a contract that guaranteed attendance I don't think this is actionable.
My way of thinking is I would rather pay for a few no shows then not have enough for everyone if we have a crasher or two. But that’s just me.
I'm not a lawyer either, but from what I have read, the only important question here should be:
"Did the guest have a duty to either attend, or to call and tell that he isn't coming?"
As I can't imagine that the child (or his parents) signed a contract in order to attend a party, the mom in question should simply plan the events better, and tolerate the fact that when you deal with a bunch of people there will be issues. Always.
The lawsuit should not be for non arlttendance but for lying on the rsvp. That’s where the loss occurred. But I don’t think an rsvp constitutes a contract.