The messages were probably deleted because your user opted to delete his account. Re-registration can be done using the same email address, though this will not restore the deleted messages.
Normally, the registration confirmation email will appear within moments of the re-registration.
Ask your user to check the SPAM folder on his mail server. (If he uses a local email program, such as Outlook, Windows Mail or Thunderbird, he will probably need to logon to his ISP's webmail service and check for it there.)
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.