The BR inspector pointed out that it is not in the realms of impossibility for the iPad / iPhone to fail
Indeed but the law does not call for redundancy or fail proof systems. A paper map can be sucked out of the window. The law requires two spark plugs per cylinder but not two paper paps in the cockpit.
Now, you might care to argue with the BR, that the guy exceeded his remit
Indeed. The inspector was clearly telling BS and had little clue but in this particular situation, I would just say "yes officer, thank you for telling me, I did not know this before" and say to myself "what an..." and move on. Nothing happened in the above case which shows that he did not have a point, just wanted to feel important.