It is possible that he has access to natural spring water with an appreciable flow. Amount of fish that you could raise in that small of a pool would be, as CB1 mentioned, determined by flow rate. Essentially it would be a wide short raceway. Generally the higher the system is loaded with fish the more potential ther is for all sorts of problems.
Absolutely Bill. Not only is temperature a limiting factor but so is dissolved oxygen and ammonia, nitrites, and to a lesser extent nitrates. If there is enough flow all of these things won't be a problem. If possible using gravity to aerate the incoming water to saturation would be a plus.
