if he just hooked a 12v charger up to a 6v battery and charged it. and nothing onthe tractor was on.. then the battery is the only casualty.
slow turn over?
is the new bat charged? 6v bats don't move often and setting on a shelf 6m may have depleated it.
charg ethe 6v batter. while it is charging.. clean up the bat cables and terminal ends and ground point to chassis.
then try her again.
unless there is something else to the story we havn't heard.....