Did my girlfriend delete her USB disk? My girlfriend says she dragged the USB disk from the sidebar in finder into trash and  I have since emptied the trash.

Is this plausible? How do I show/display a USB drive using the command line or finder?
Dragging a drive into the trash just means "eject", not "delete contents". Since USB can't be physically ejected, it just unmounts it. Emptying the trash has no effect in this case.

Unplug it, wait a few seconds, plug it back in, and it should show up.

If you don't want to unlpug and plug it back in, see hdiutil attach for CLI solutions, or /Applications/Utilities/Disk Utility.app for GUI.