You can SYNC it to your account, and all your friend's media will be erased. You can't re-register it. When you're done with it, give it back to your friend. He will sync it back to his account, and all your data will be erased.
Warranty should start from date of purchase - you never need to register it with Apple just set it up with iTunes. You may get the prompt to register each time you connect to iTunes but you don't have to - I've had mine a few weeks and haven't registered it or numerous AppleTVs