I don't have a problem with it either. It's a fact of life that people don't always get invited to things. I would understand that the parent can't invite everyone or that the child didn't want my child there for whatever reason. I would just explain that to my child IF they even noticed. I would take it as an opportunity to try to build my child into someone who deals with disappointments in their life appropriately.
It has happened to my eldest, and i simply explained that the parent couldn't invite everyone, and said, that he hadn't invited everyone to his party either. He accepted that and moved on.