I would change your gear a little bit, instead of the Apogee Duett I would use the new UAD Apollo Twin , but only, if your Macbook Pro is coming with Thunderbolt.
The Apollo Twin offers near-zero latency and you can use the UAD-Plugins, too. It's perfect for recording guitars and vocals and it's very easy to handle. Because of the excellent preamp-section (mic/line/Hi-Z), you don't need a channelstrip for recording. If you want to record vocals or acoustic guitars with the Apogee, you urgently would need one.
Why would he need to upgrade his interface to record vocals or acoustics????