My experience...
I signed up two friends this week for the refer a friend.
The first one i signed up then he purchased a ticket.
The second friend purchased a ticket through a non/NWA web site and the next day i signed him up to World Perks Program and logged him in as my refer a friend. NWA said this wasnt a problem and i would get the miles. I'm hoping the CS person was right and knew what she was talking about, only time will tell though.
hope this helps
lucas