When we had high temps like this (42 degrees celcius) my horses were hosed at lunch time and again at about 5 PM.
Sometimes it is cooler to actually keep them in their stables if the barn/stables have been built to maximise the cooler air rushing past.
I basically don't ride during those days. If I don't have the energy to throw a saddle onto his back, there is no chance he'll want to go for a ride. And I'm not going to force him to work in the heat as there really can be no benefit and makes riding a chore.