If your friend doesn't use your affiliate link or coupon during registration, then you will not be credited for the member referral.

If your friend already used your coupon or affiliate links during member registration, then he/she doesn't need to use your coupon or affiliate link during checkout. You will AUTOMATICALLY be credited for the purchase if you have already been credited for his / registration.

That is, unless your friend explicitly uses someone else's coupon code during checkout – this is the only time when you will not be credited for his / her purchase during checkout.