It's not difficult to get confused when plumbing. I'd definitely have the guy come back and straighten it out. You can check yourself just for your own peace of mind. The cold water should be flowing INTO the cold side of the hotwater heater... hot should go out the H side. It should be marked on top of the tank near the fittings ...H on one and C on the other. Simple test is to run the hot water and feel the pipes then look for the H or C. I've actually had one to run backwards when a mixer would be left open and valve was put on the hose pipe bibb. (at a tub for bathing dogs). Too, it doesn't sound like it..but a little warm then cold sounds like one element is out in a hot water heater.