I would check that this was the case. I know when I did my one and only referral a couple of years ago, the discounts did not appear for a couple of months. I can't remember the explanation I got, but it must have made sense at the time - I don't take prisoners on this kind of issue.
Check next month, and also check on your account that it is showing as a 'live' referral.
Sounds like something may have gone wrong with the referral system. I’d ask your friend to call 0800 432 0200 urgently. Make sure they have your account username so the referral can be manually added.
I say urgently because I believe a referral can only be added up to 90 days starting from when the referred account was first signed up (From reading previous posts on here about this subject)