From what I can understand, you do have a backup of the database from before deleting that user, don't you? Please correct me If I am wrong. If this is the case, just add an entry to the users table with the data of the backup related to that specific user.
If this is not the case, it is practically impossible to recover the entry (you would have to know the password hash... etc). I suggest you to go visit the Official TestLink forums, where I am sure they will be able to help you.