He’d get better results if he tried to do the same thing with the waste water from shower, dishwasher and clothes washer drains instead of the incoming water.
However, I have my doubts that the efficiency numbers work out, or enough to justify the cost of the equipment.
I know a guy who installed a drain-water heat-exchanger; from the temperature deltas and flow rates he said he was seeing, I figured he was getting an effective 7kw from it using the warm drain water to preheat the replacement water flowing into the water heater.