Wait, are you opening it from the Start screen? That switches by default instead of opening a new instance, but for desktop apps there's an app command (right click) to open a new window. I'd still recommend shift-click from the taskbar though (same as in Win7).
I think this may be an issue that will be popping up a lot, people thinking that everything has to be started from the start screen when everything works just as it always has from the desktop (minus a start menu, which is actually the start screen).
I think the issue here is more that he probably didn't know how to start a new instance from the taskbar in Win7 - which is understandable as it's not exactly obvious - and was used to doing it from the start menu. However, you could make a case that switching to an existing instance is the more mainstream expected behavior and starting a new instance is a bit more advanced, so it's ok for it to be tucked away a bit more (but convenient enough once you know how).
