Macbook Pro 13" with Retina Display: Black Screen
There is a small possibility that the video cable is mis-seated and simply unplugging it and re-plugging it in might help.  ifixit.com can help you with the take-apart.  It is not difficult, just make sure you ground yourself and disconnect the battery before touching anything.

Has the machine been dropped?
Has anything been spilled on it?
Is the LCD panel physically broken?

To connect a Macbook Pro to an external display you simply need the proper video adapter.  Either Mini-Misplayport to VGA or Mini-DisplayPort to DVI, depending on the monitor.  Most monitors have many options, including both VGA and DVI.

If the laptop and monitor are new enough you can use an HDMI cable.

To further diagnose your girlfriend's broken laptop connect it to an external monitor.  If the lines appear on the external monitor then the issue is the Logic Board.  If it works fine then you have a LVDS cable or LCD Panel issue and will need a new display clamshell.  iResQ.com does cheap display repairs.  No, I don't work for them. :)