Yes, provided that the order has not yet been dispatched.
Login to your Friends account. Go to “My orders” (1), select the order you want (2) and you’ll see the “enter voucher” option (3).
Simply enter the code and click “redeem”. (4)
You’ll then get a brief overview of the value of the goods and your discount. Click on “OK” (5) and the voucher will be added immediately.