once you plug it into the USB port and the itunes recognizes it click restore ipod to factory settings or whatever that should delete your friends music...and if you wanna put his music on your itunes let me know before you restore it, also thats a hard process but let me know and ill tell you
eh not that hard, copy it off the ipod, add it to itunes and its all renamed....but ya its locked because you have ro register an ipod to a computer...hopefully restoring it works, if not there are some other things to try.
just connect your ipod, go to edit-->preferences-->iPod, then it should have a check box that says manually update all songs and playlists. then you can put your own songs on and also keep some of his. its easier than restoring it.