First, you may want to talk to the girl in question and then to her mom. As far as how to react, just remain composed, not angry but rather ask her what made her do that.
She could have either wanted to surprise the rest with her "breakfast" or she has a streak in her that makes her defy rules. Perhaps she had felt that if she does this in someone else's home, she won't get in trouble.
Kids are also prone to making mistakes. You may want to explain to her why it may have not been appropriate/safe to make the breakfast.