my collection (47Gb) doesn't fit on my 40Gb iPod

what you need to do is use the iPod as a removable drive (enable this in iTunes BEFORE you give it to your friend) and get him to copy his collection onto the drive as a folder (not via iTunes).
then take it home and copy the collection onto your hard drive. then delete the files from the iPod and disconnect it.
Then go into itunes and import the collection into your existing collection. (set it so it copies the file too, not just adding it to the database - my be the default).
then delete the copy of the files from your hard disk and reconnect your ipod to syncronise the files to it and your done.