a) If your "friend" is actually you - nice try, but that's not going to work.
b) The user interface may need some time to update, be patient.
c) Your friend probably didn't click the link. Just because you posted a link, it doesn't mean people clicked it. Git gud at marketing/hyping.
d) Your friend probably didn't register for The Sandbox or were already a member. Or they deliberately removed the referral code from your link before using it.
e) Your friend's adblocker or browser cookie settings may have removed the referral link's cookie, meaning The Sandbox cannot tell your friend used it.