Thermite is currently using Qt but I wouldn't recommend he works with that... it's to unstable at the moment. The PolyVox demos are also written in Qt but don't do much interesting, Qt is just used to house the OpenGL widget and get mouse input for rotating the volume. Of course you could extend that into something much more interesting (some kind of voxel editor perhaps).
If you do want to download a Themite demo to show him you can get them here:
http://www.thermite3d.org/joomla/index.php?option=com_content&view=article&id=15&Itemid=24The November 2009 demo is probably the best one, and earlier demos may not use Qt.
Otherwise, you can always find some interesting open source Qt applications at
http://qt-apps.org/