A pixel, in terms of a monitor, is a physical unit; thus, it cannot be increased or decreased. A monitor with a resolution of 1680x1050 has 1,680 pixels across it's length, and 1,050 pixels across its width.
A 1680x1050 monitor can display resolutions lower than 1680, but it cannot display anything higher because it's just not physically possible to do so. You can play something in 1080p, but because you lack the hardware, it's going to run at 720p. It still looks very sharp, but it's not 1080.
I'm still not real clear on what you saw your friend do, but whatever he did, he did not add pixels to the monitor.