Another friend joined but forgot to use the invite link before doing so. Is there a way to add me as their referral on their now-existing account, fixing the mistake?
He deleted account and created a new one using the referral link, but doesn't trigger the "recruited" threshold; no report saying friend has joined world, not added to list of recruited players, etc. Something is seriously broken with the recruitment system.
Ask your friend to submit a ticket he believes something is wrong. It is honestly hard to guess as to the reason for it not working. Submitting a ticket in-game allows the staff to actually look into the matter to see if there is an issue.