I don't use itunes, but I think itunes just saves a playlist and not the actual files. So the files you dropped to your friends itunes just made a bunch of shortcuts to the folder you had saved them to on you computer. When you disconnected from his computer you broke the link and his itunes can't find the files.
You'd need to reconnect to his computer and transfer the files to a folder on his computer, like his "My Music" folder. That is if you still have them. If not you'd need a file recovery program to try to get them back.
Deleting Files From Limewire Library-don"t-File Recovery advice. How to transfer music to my iPod
Other members have more experience with itunes and may have better advice. Hope this helps.