Good on you girl! Moan all you like.
As far as I'm aware leaving your engine running when you first start up in cold weather is considered to be determental to your engine, and battery. (Not to mention the noise and air polution!). All of the cars engine parts don't get equally warm, when you aren't actually driving - this causes condensation which causes corrosion, and also the water (condensation) mixes with the oil in parts of the engine lubricated by oil. It also means that expansion due to heat doesn't happen across all engine parts equally, causing engine damage (because the parts don't quite fit right!). Also the oil doesn't pressure up properly for the first few minutes if stationary, so parts of the engine that should glide due to oil, only have residual oil (and condensation), which causes engine damage. When only 'ticking over', the engine takes a very long time 20-30 minutes to 'warm-up' properly, where as if driven it will warm up in about 5 minutes or less.
Leaving an engine running, would take hours and hours to charge the battery, and if he turns on all the heating fans, and heated screens then if will more likely flatten the battery, than charge it. Repeatedly doing this without taking it for at least 30 minute drives will damage the battery.
Diesel doesn't freeze until about -40C. Again, my understanding is that it can cloud and form a gel when exceptionally cold. My understanding is that additives are added to diesel fuel in winter months to drop the gelling point from around 0 to -10, to around -20C. Small amounts of water freezing in the fuel system and fuel filter is more likely than diesel forming a gel. In which case a hair dryer to warm the fuel system would be more appropriate than trying to start the engine!
The best thing any of us can do for our vehicles in the cold weather is to put on plenty of clothes, scrape all ice/snow off the car, and around where your vehicle is parked. Start the engine and drive away, as soon as you start the engine. Then stop as, and when necessary to remove layers of clothes as the car warm up. It might steam up on the inside when you jump in, so you may need to be stationary for a short time, but 2-3 minutes should be the maximum 'idle' time, before you start doing your engine serious damage over time.
Oops, I seem to have written an essay - but now you have some ammunition to fire back at him!! Also you might want to check if what I've said is true, because it's just stuff I've picked up over years, and can't guarentee it's correct!! (But it sounds good hehe!!)