In dealing with victims suffering from dehydration, we usually gave them access to a canteen and let them drink at their own pace. In the usual desert scene, dehydration, plus other injuries, was common. If one did a careful survey, probably a good percentage of the rescuers were dehydrated to some degree as well.

Personal experience - On one occasion we were investigating an archaeological site that was reached only by fairly involved technical climbing. We stayed overnight, descending later in the morning. A companion, realizing we would be thirsty, climbed partway up the route with canteens.

He handed me a full quart, which I drained without pause. I put it down and was still thirsty.

You want to supplement the water with ORS, salty crackers, dilute Gatorade, etc. in moderation. In extreme states, especially if the victim may be facing surgery, establish an IV line.

BTW, you don't have to be in a desert to experience severe dehydration.