I sure hope he's insulated that pipe!
I wonder if he would consider a pre-heater tank, like in
this BuildItSolar project. The water that's entering the hot water heater is pre-heated to prevent a drop in tank temperature. It sounds like his system stops standby losses, but I can see a situation where hot water is demanded but there's no water is flowing from the boiler, or even where the thermosiphon works in reverse!
That never happens, right? Or he has a check valve?