Stuff the battery back in so that the terminals are on the same side of the box that the cables are on! Unless he has abnormally long cables it will only fit in the correct orientation.
Ok, let's do this the right way: get a multimeter and carefully determine which terminal is positive. Then pick a cable at random and flick the end of it against that positive terminal. If nothing happens, you made the right connection. If there's a nice arcing spark and "FZZT" sound, that would be the negative cable.