Yes, referrers don’t always have to be existing customers. A visitor to your website, for example, can register on your referral landing page and successfully refer a friend.
This can be beneficial when visitors aren’t currently interested in your products or services, but know someone who will be. The reward offered on their first order also means they’re more likely to become a customer in the future.
Once a customer signs up we then consider that customer to be a Referrer (existing customer) even if they haven't bought from you previously. This would therefore negate them from being able to be referred by another friend and claim the introductory offer.