It depends on what you mean by "cheap." They're a bit pricey for a used player (most good Rockbox players have been discontinued), but the Sansa Fuze (original, not +) and Sansa Clip Zip are great Rockbox players and both feature a MicroSD card slot, good sound, and all the benefits Rockbox brings.
Those benefits include many different themes/fonts, more playback and equalizer options, support for a massive variety of audio formats, easy-to-use bookmark function for audiobooks, and robust playlist support. It really depends on what sort of features he's looking for, though it's possible the original Sansa firmware will suit him just as well.
The Rockbox manual details all the features.