The referred member has to earn a certain number of points - it was 60, not sure if that has changed. Once the new member reaches the milestone the referrer earns 100 points.
Actually - looking at the email I got from MyPoints in June congratulating me for earning 100 points in the refer-a-friend program, it says "because you referred Member Referred To Mypoints during our Valentine's MONTH promotion, you EARNED 100 MORE POINTS which will be credited to your account separately!*"
That means that it either took 4 months for my friend to earn 60 points (don't you earn a chunk of points for completing a survey?) or it took MyPoints 4 months to credit me.