I don't think you can get the furni back the person who hacked your friend used a "dummy" account and transfered the furni he hacked onto his main, unless you know the hackers main name, and find someone who can and wants to spend a few hours hacking for virtual furni, But good luck on getting the furni back.:/
Its almost impossible to hack somones account without scamming them, or knowing their password. Because all usernames and passwords are excrypted in coke studios servers, so even if somone were to get into it, they wouldnt be able to decode it.