The GS-10 doesn't record anything itself... it converts analog audio input into a digital signal that it sends out via S/PDIF or USB, or it converts that digital signal back into an analog audio signal for its analog outs.
To me, converting a digital signal back to analog and then re-digitizing it makes no sense if you're recording direct anyway. If you can keep things in the digital realm, you can always EQ or process some "warmth" into the sound while keeping the original signal as clean as possible.
Whether he uses the digital or analog outputs on the GS-10, there still won't be much analog "warmth" in the chain, unless he plans to run the analog signal through a tube mic preamp or compressor before taking it into the computer.
If your cousin follows your advice, he'd be sending his signal through two additional conversion processes (digital-to-analog on the GS-10 output and then analog-to-digital into the computer). Personally, i would try to minimize the number of analog/digital or digital/analog conversions in my signal chain when recording direct, and i'd go either the USB or S/PDIF route, rather than potentially losing information in the extra conversion steps.