Glad to know it wasn't a big problem.
I think that is one of those "rules" that's written down somewhere - sort of like "The likelihood of the bread falling onto the carpet with the peanut butter side down is directly proportional to the cost of the carpet ......"
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.