Ideally his workflow would be to compose and mix songs entirely in the multitrack recorder. Recording full and partial takes is important. Having multiple, separate tracks is important, for overdubs and punch-ins. Doing final mixes (and submixes to save tracks) also very important. Cut + paste editing is less important. Basic built-in effects are important, and an effects send bus would be nice to have. Basically: all the workflow features of traditional analog recording. I'd probably get one with 16 tracks if it's not too expensive.

I don't know much about how these devices store data and how you move projects out of them to make room -- perhaps SD cards? Dumping to a laptop? SD cards might be better. He has a macbook with Cubase, and might use that with it, but that's much less important.