If your friend can live without digital-in/out I´d go for the Marantz PMD620.
Check my 9h55min testrun with 2 rechargeables(2700mAh) at 24bit/48kHz.
Using line-in will increase recording time as no internal mics are drawing power then.
It has the size(10cm x 6cm x 2,4cm) of a cigarette-pack and runs with 2 1,5V batteries or rechargeables.
MP3 recording is limited to 192kbit though on this machine.