well, he got it used about 5 years ago and it didn’t have anywhere near the same look as the mac mini did then. So, maybe, in 2005, he got a monitor from 1998-2001 maybe?
I'd bet 2000-2004 at most. That would be the brief period Apple flirted with the Apple Data Connector: USB, DVI and 100 Watts of power all in one cable. That's so you could hook a monitor up to a computer with one cable, and plug your mouse and keyboard into the monitor. It cut down on clutter immensely. Apple had to abandon it as monitors started sucking more than 100W.
Overall for your friend though, dumb move to buy a computer and monitor without bothering to see if they actually connect to each other, regardless of manufacturer.