This is an interesting idea. However, I don't think this has to do with dual link. As my understanding is dual link just ups the clock cycle or transmission rate.
I think the post below is a more likely scenario. Although I could be wrong.
http://www.3dgameman.com/reviews/aiw_dual_display/aiw_dual_display.html
The basic idea, in the above article, is the DVI-I standard has analog and digital on different connectors. Using a splitter you can split the analog and digital connections. If the video card supports it, you should be able run "true" dual screens. There are several issues. You also need a VGA monitor or one that possibly supports DVI-A(which is maybe how your friend got it working from looking at the cable?), and a DVI-D monitor. DVI-D is just normal DVI. It also looks like a lot of people had problems with monitor detection on the VGA connector. The workaround was to deactivate a certain pin on the VGA connector. There are adapters it seems, others just ripped out the PIN instead.
Read the post above and ignore the flamewar that sort of surrounds it. I would suggest skipping to like page 7 or so.