My listening room at home is just about the same size as your friends. My current system there is Soliloquy 5.3 speakers which are 90db floorstanders, paired off with George Wright Mono 10's which are 10 watt monoblocks (push-pull 2A3 based amps). Plenty of power for the size of the room, but as has been said, don't expect chest-pounding bass. My system can play at very 'satisfying' volumes and remains engaging at low volume as well. YMMV depending on which amp and which speakers you end up pairing off. Being a 2A3 based system it is all about midrange and leaves a bit to be desired in the low end. I've tried in the same room pairing off 8 watt 300B SET amps with 89db Silverline SR-17 monitors. Though they did sound wonderful at very moderate volumes, this combination did not work at all well when the volume went over about 88db (I'd guess). Numbers don't always tell the whole story. But I think the general speculations and expectations given his taste for jazz is a good bet for a nice solution if done with care and consideration for synergy.
Marco
Marco