You should have gotten an e-mail from your friend, notifying you of the gift. Note that this e-mail/gift MUST be to your Apple Store e-mail.
Once you get the e-mail, there should be a button within it to "Redeem" the gift. Press the button, you'll get directed to the App Store where you'll sign in. Once you sign in, the app should begin to download.
Hope that helps.
Marilyn