You can't if you haven't received the email. You've checked the spam folder as well as the inbox on the email account that your friend sent it to ? If it's not in spam then ask your friend to try resending it (and double-check the email address) : Resend iTunes Gifts - Apple Support
If the resend doesn't arrive within a few hours of resending it then your friend could try the 'report a problem' page to contact iTunes Support : http://reportaproblem.apple.com
Or he/she can try contacting iTunes Support via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption