Did my girlfriend delete her USB disk?
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.