This can be done in 2 ways. First you can always register new users with protected usernames as admin in your XenForo ACP. Second, if you like to allow him to register himself via the regular registration form, you need to manually work with your XenForo database to accomplish it. First you can check if the deleted username really belonged to him if you ask him for his email address and compare it with the email address shown beside the deleted username in the database table of our add-on. If so, you simple can delete the protected username from the database table of this add-on to allow a new registration with it immediately. There will be a search and delete feature in one of the next versions of this add-on.