A screw had fallen out on one side of the metal clamp that holds the battery in place.
The battery was therefore free to move out a little bit and in doing so pushed one of the surrounding wires against the side panel so the insulation was gradually worn away until the wire shorted against the side panel. This in turn blew the fuse related to said wire. All is now fixed and the bike is now running again with lessons learned.
He secured the clamp back in place with a cable tie which I actually think is a better option as it won't fall out.
I shall also secure all the loose wires in that area from movement by simply sticking them to the battery with gaffer tape.
@Clubman:
Glad you have found the real source that shorted the fuse.
Recently, I had replaced the screws that hold the clamp (to keep battery in place) with Phillips head screws so that I can use one screw driver to open the battery clamp and the batter terminals (I have Yuasa battery that has Phillips head screws on terminals).
At that time, I installed LOCKING washer (split washer) on the screws, so that the vibrations of motorcyle may not loosen the screw. These locking washers act like a spring and push the screw head - hopefully that keeps them tight.