Hi,
A little unleaded gas mixed into the diesel fuel is not really all that bad. I was reading a book on diesel cars back in the early 80's and they stated that if no "Winterized" diesel is available, you can add about 1 quart of unleaded fuel per 20 gallons of diesel to winterize the fuel, so it will not get to thick at 20F to start the vehicle.
40 gallons in a 120 gallon tank is a huge amount of unleaded gas, and diesel do not run well on that much gas to diesel mixture. I guess that the generator ran as long as it had real diesel in the fuel filter, until it stated getting the gas/diesel mixture.
Cars with high compression engines "Can" run on a little bit of diesel mixture in the unleaded gas, however if to much, the fuel burns so much slower that it will act like 130 octane, and barely run at all. High compression (such as 9.0:1) will run at a high enough temperature to keep it running, but probably will not start cold at all. I would not try it on a fuel injected car though. I did run some diesel once in my 1975 F-350 with 8.5:1 compression. I put about 1 gallon into the 19 gallon rear tank. It ran fine, did not ping anymore either. But that was a massive 7.5L engine with a carburetor about the size of a breadbox.
Anyway my guess is that they drove about 250 miles on the "New" fuel, and will not have any problems. I think they will be fine. But also check the fuel pump for leaks. If you don't see or smell anything, it will be fine too. Unleaded fuel is much thinner and can "find" small leaks, and work it's way through the seals.
By the way, large refineries send diesel and gasoline through the same pipelines all the time. First they send several thousand gallons of gas, then change to diesel, jet fuel, then back to gasoline. Usually they pump one fuel for about 2-3 days, then change over. The mixed fuels from change over usually goes into the "Fuel oil Tank" - sold as home heating oil. Home heaters can take a mixture of unleaded and jet fuel or diesel and still run normally.
Fred.