I bought a app store voucher but my mate hasn’t received it yet

If you mean that you gifted him an amount or an item (he's checked the spam folder as well as the inbox on the email account that you sent it to ?) see if you can resend it : Resend iTunes Gifts - Apple Support

If you can't resend it contact iTunes Support : http://reportaproblem.apple.com or https://www.apple.com/emea/support/itunes/contact.html