@Heidi-Edith0 You need to explain to this guy that he can't just have more people stay than the number he booked for. If his friend wants to stay, he needs to be added to the booking. And if you charge per guest, make sure he understands that there will be an added fee.
Also, you seem to have a sense that this guest will have even more people over, so make sure he is aware that his booking will be cancelled if this should happen.