Your guest would have been notified the same way as would be notified for for example a reservation request. If your guest has chosen to not receive any notifications, the only way the guest would have known, is by logging on to his account. But in my opinion, if a guest chooses not to receive any notifications, and come back to you with his request only a month after he stayed with you, it is his responsibility. You made him an offer which he did not accept (for whatever reason). For me that would be it.