Use it like this:
Run >NIL: JoyMouse 9
Reading the docs to JoyMouse (by J.Potter) it says a left mouse click and a firebutton press will quit it. So if your friend presses his firebutton and you do too (simultaneously) then it will quit (whilst Workbench is sill running at least). It shouldn't quit whilst in WHDLoad, so shouldn't be an issue.