I haven't done a flight to Australia in a long time, but as far as I remember we spray before pax boarding, about 10 min or so, I can't remember what type of cans but by the time the pax board the whole thing is gone, I doubt that your patient will suffer, don't forget that the door is open also, so there is some fresh air coming into the cabin.
He can board last if he wants to make sure that less residuals of the spray is left in the air.
No more spray into the UK, that has changed about 2 years ago.
I hope that this helped and that your patient enjoys the flight.
