So how are you going to get 100s of gallons of diesel delivered in the middle of winter ?
I'd imagine the same way he'd get propane delivered? By a truck.
Where are you going to store it ?
I'm going to guess a diesel fuel tank. Likely double walled to meet environmental compliance. Depending on how much he wants to spend on this project, he can likely get a sub-base tank to sit under the generator... but he's looking at an industrial/commercial unit at that point.
Diesel and really cold temperatures are not good friends.
For colder climates, I'd pick a diesel over propane any day. As the temperature drops, propane fueled systems lose fuel pressure, as there is a direct correlation between temperature and pressure. (Ideal Gas Law at work: PV = nRT)
As fuel is drawn from the tank, the tank temperature will drop due to the fuel delivery to the engine, and make it even worse. So you need a large tank with a lot of surface area to absorb the ambient heat to keep the pressure up.
One way to deal with his is to bury the tank underground and use geothermal heat to keep the tank pressure up. Another way to deal with this is to use a liquid withdraw system that pulls liquid propane from the tank, and does the liquid/vapor conversion next to the generator. Using a propane vaporizor that is connected to the generator's liquid cooling system will help prevent the vaporizor from freezing up, or you can use an electric powered vaporizer... but that pulls power from the generator so you'd have to up-size the generator to account for it.
For what it's worth, I've been in the power industry for about 30 years. We have thousands of facilities all over the US and US territories... and some are in some very inhospitable conditions. Mountain top sites, prime power sites that provide 24/7 operations in Alaska that don't have access to a grid, you name it. Our prime power sites in Alaska are all diesel powered. We barge in 30k gallons of fuel per year at one site alone. We've looked at propane as a fuel source for those sites, and rejected it on numerous occasions.
We are currently researching alternate sources such as wind and solar... and that has led us to look more into running high voltage submarine cable from a grid to our sites as we cannot rely upon wind or solar in the cold winter climates at some of our sites.
Another reason for propane generator. It will push you to solar.
I'm not quite sure what one has to do with the other... I'd push for solar and a large battery bank for the day to day stuff. The generator could be started and ran when he needed 'big power' for welders and stuff like that. He could also have a battery voltage monitor that would automatically start the generator when the main batteries hit a certain low voltage limit. If the solar panels keep the batteries charged, it doesn't run. If the batteries drop below the voltage threshold, the generator starts and runs to recharge the batteries. Set the run time for however long it takes to recharge the batteries, then have it shut down.
I just ran a comparison between two 30kW generators. One LP, the other diesel. At no-load, the LP generator pulls 34 cubic feet of gas/hr... just under a gallon. (36.39 cuft = gallon.) The diesel burns 0.8 gallons/hr. At full load, the LP unit burns 164 cuft, or 4.5 gallons/hr. The diesel burns 2.6 gallons/hr. A propane tank should not drop below 20% full to support a generator, so a 500 gallon LP tank would be able to provide about 400 gallons of run time. So a 500 gallon LP tank would be equivalent to a 230 gallon diesel tank.
The cost of residential propane is at about $2.36/gallon, whereas off-road diesel is at $2.85 or so? So to fill up the propane tank, you're looking at $1,179 vs. $655 for the diesel.
Granted, he might not need that much power... perhaps a small home/residential generator would work rather than an industrial model. But if it was me, and I was relying upon it for regular use, I'd go industrial in a heart beat. Something like this 15kW Kohler with a 209 gallon sub base tank and an all-weather enclosure would be pretty sweet:
https://kohlerpower.com/en/generators/industrial/product/15reozk
It burns 0.4 gallons at no load, 0.7 at 25% load, 1.1 at 75% load, and 1.4 at full load. With a 200 gallon tank, that's a lot of run time... somewhere between 149 ~ 300 hours, depending on the load. If he only ran it an hour or so a day to recharge the main battery bank, that's a few months worth of fuel.
So from a cost perspective, as well as an installation and reliability standpoint, diesel is the obvious choice to me... but there may be other circumstances that the OP hasn't brought up that might sway that one way or another.
Just my two cents, adjusted for inflation.
Mark