why cant I sync music from my iPod to my iPhone?

A friend of mine got the new iphone 4 so I got his old iphone 3. I am trying to pass all the music from my ipod onto my iphone but it wont let me sync ANYTHING whatsoever. I am using the new version of itunes but on a different friends computer and I tried to sync some of his music onto my phone but it wont work. And i know its possible because I have synced some of his music to my ipod before and it has worked well. so what do I need to do to transfer all my music from my ipod to my iphone without deletting anything?