I dont think I would expect a modern diesel (Or even an old one, like mine! ) cabin heater to blow "Hot" at idle at this time of year, They simply dont produce enough waste heat!

My first port of call would be to change the header tank cap (In case it has got a bit weak, as they do...), change the thermostat (Might help the heating bit) and "educate" the owner about not topping up beyond the "Full" mark on the header tank and see where that takes you.

HTH

Paul