I would be checking the boy very carefully for spots just in case the sore throat isn't strep since the doctor didn't test. He could very well have the same as the others. Just a little bit of a coincidence that he gets a sore throat just at the same time others get sick with a disease that causes a red sore throat.
He has already been exposed so it is too late to prevent that. I would be staying open and especially since health unit/telehealth said okay. If she chooses to keep him at home that is her choice but you are open and payment is due.