The question is not asked very well, you man mean that this individual is the patron.
You should call 911 (or any emergency service in other countries) if the individual loses consciousness because it is a sign of a severe dehydration.
While waiting for the ambulance to arrive, you should keep the individual in a cool place with shadow, and give him water to rehydrate. To complete your sentence: An individual who has been in the hot sun all day complains of weakness and nausea. they are sweating a lot. you have someone call 911 right away if the patron refuses water.