In reply to:
No he put my email address as the referer
Maybe he made a typo?
Only Dreamhost could give you a real answer on it, so you could submit a ticket.
Other guesses:
If the domain was ever hosted at Dreamhost, I don't think you'd get credit.
Not sure on this one, but there's a "How did you hear about us?" box right over the box on the form where you enter the promo code or referrer email address. If he put it in the other one, I don't know if they'd automatically credit it as a referrer or not.
Also not sure on this one, but if he clicked a promo code link (or got cookie stuffed), maybe that over-rides it. I doubt that's the case... since it would reward cookie-stuffing over someone manually typing in who they want to get the credit.
If you find out through support, let us know what the problem was. 