Just test-drive several before he buys. Paying close attention to the fingerboard and body as well as the sound. Mandolin fingerboards can be really tiny and he should be sure he's getting something that will be comfortable, not sardine-like. Mandolin requires a lot of finger pressure so unless he's an ironman, don't get one where the strings are set very high off the board. And, is it comfortable to hold in general, and does it sound good?