If someone is experiencing heat exhaustion you should immediately get the person out of the sun and heat and into an air conditioned building or shady area outside.
Then lay the person down or have the person lay down and elevate the persons feet slightly.
Remove or have the person remove any heavy or tight clothing they may be wearing and have the person drink some cool water or any other non alcoholic or caffeine free beverage if no water is nearby.
Although cool water is the best thing to drink to hydrate a person who's experiencing dehydration and heat exhaustion.
If the heat exhaustion is causing the person to Vomit, have seizures or the person is becoming unconscious then call 911 immediately.