If you want to delete the songs from iTunes and leave them on the iPod you have to enable it to "Manually manage songs and playlists."
You're right though, unless you can find a friend's in which to plug your iPod (It will come up with a message asking if you want to associate your iPod with your friend's library - click no, then go into the iPod preferences and change it from Automatically Manage songs and playlists to manual control, you can then plug it back into your computer) you will lose your songs.
This is discussed in the iPod user's guide - the iPod comes from the factory set up to auto sync with your iTunes library. This is to maintain utmost ease of use so that you are only managing one library. However if you have a small HDD you can set it to manually update and manage it as a separate library.
So, unless you have a mate's computer handy you're pretty much stuffed. Sorry, but you should have read the book first. Hope your music's backed up.