If you are basically “sharing” their ipod then I don’t see a problem. The same way you can copy onto your own ipod, you can copy onto theirs if it is for your own listening. This should be no different than putting your CD into your friend’s car when you are driving together.
Think of the music lomdishly as a physical CD and act accordingly. If you lend a physical CD to a friend you obviously can’t listen to it on your own CD player while he has it so if you put music on your friend’s ipod you shouldn’t listen to the CD on your own away from that friend.
I had a friend who didn’t have a computer but had CDs and an ipod. He put his CDs on my computer in order to sync his ipod. I kept his music separate from my own and only listened to it when he was there.