Ask them to try again with dummy email accounts. (Easily obtainable from sites such as Hotmail, Yahoo, etc). They should make sure they are following the instructions exactly, and that they\'re picking a unique name (i.e. don\'t try to register the same name as they picked before!).
If it works, they can go into their Profile and change the email address later.
Usernames/emails are only kept if the account is successfully created. Thus they should be able to either 1) Use the original accounts, or 2) Use the original name/email address, depending on whether the registration worked or failed.