Do you know approximately how much he put in - 1qt, 1gallon, etc.? How about how far has it been driven since?
Ok, windshield washer is mostly made up of water and the small amount of chemicals that make up the cleaner compound will not harm any of the internal parts in the engine's cooling system. A quart isnt enough to significantly lower the boiling point or raise the freezing level of the coolant/antifreeze.
Since it hasnt been driven, it is still in the resevoir. If you really want to, although not necessary, you can drain the resevoir, or even use a turkey baster to suck out the fluid. But as I saind, it wont harm the system being left in...