For possible legal reasons I will make my comments general. They do not refer to any bookmaker in particular.
I do not believe it is illegal for a bookmaker to give odds of 1000/1 about an even money favourite to anyone the bookie wishes to.
But it is a breach of a bookmaker's licence to record such a bet if the bet has not actually been made.
It seems obvious that if a bookmaker does make such bets, then the bookmaker expects to claim the losses on tax.
However there is also anti tax avoidance legislation. If the Tax Department becomes aware of such actions by any bookmaker it may be in a position to disallow the claimed losses.