There are two, at least, methods of updating the iPod with music tracks. One is automatic and is probably the one that your friend is confused over. Using this method, the iPod will synchronise itself with the iTunes library of the machine it is connected to. This will replace the entire iPod library. If the track is not in the Library of the computer, it will disappear from the iPod.
The second method is the manual way. (didn't he play inside left for Real Madrid?)
Using this method, the tracks on the computer are manually dragged onto the iPod. Nothing is lost anywhere.

To set this preference, go to the iPod preferences and set the ipod to Update Manually

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