If you sent on the site, try in game. If you sent it in game, try on the site :)
Invitations can take a few hours to be delivered, though your friend might find that it's already waiting for him in account management. If he goes to Battle.net, chooses his WoW account, clicks Referrals & Rewards at the bottom, and then Scroll of Resurrection, he should see a 'received' tab.
Any SoR invitations that have been delivered will be listed there and he'll be able to accept one of them even if he hasn't received an email about it.
More information can be found under the SoR walkthrough:
If you have and you are unable to claim the scroll, then please contact our Account and Payment team who will be more than happy to take a look for you. :)
Details on how to contact them if you need it, can be found here.
Account and Payment Issues