A couple of years ago a man was carting two Dogues round a game fair all morning on a really hot day. When one collapsed he rushed to our stall saying it needed some food!
We soaked it with cold water from our dogs' bowls and bottles, then sent for buckets of water from the nearby lake which we poured all over, particularly on the stomach which cools them quickly. We sent the man to get his car to take them to the vet and by the time he came back we had got the dog round and it was lying under our van in the shade chewing a pigs ear. He seemed more worried what his wife would say if he had killed her dog! Dogues have trouble breathing at the best of times, as do any of the short faced (bracephalic) breeds.
If you ever see a dog in trouble from heatstroke you need to bring its temperature down as quickly as possible. Dunking in cold water is the most effective way, but soaking the head and chest/stomach is next best. Trying to get them to drink is not possible if they have collapsed.