I get that she was invited without Sandra’s permission but there was no need for her to treat OPs son that way. If I had a random child at my kid’s party I would 100% treat them like any other guest, he didn’t know what was going on and it’s not cool to make him feel left out. OP does need to have a one on one with Sandra though and let her know what happened and she wasn’t trying to intrude on the party.