Adding a playlist to a cleared iTunes library, to be synced to half-full iPod
I never ever recommend using an iPod as the only form of storage for music. Inevitably that iPod will become lost, stolen or dead, and his music will be gone forever.

That said, there are two suggestions for what you asked. The first is to place the iPod into "manually manage" mode. This kbase article from Apple is a good guide that covers the different methods for different generations of products.

The second, recommended suggestion is similar to what you mention in the question. First move the music back to a computer. You will need special software to do this, such as the cross-platform CopyToy. Once the music is back in iTunes on the computer, create this new list and sync ALL of the music over. The iPod will be wiped of all of its music and replaced with the same songs, plus the new playlist. And while he's at it, have him backup his iTunes Library so he doesn't lose his music.