This sounds pretty impossible without knowing at least the VIN.
Did her parents buy it new?
Maybe they or the dealership kept a record of the sale. Assuming the dealership is still in business and might at least have old contracts kept off-site on microfiche or digitally imaged or something like that.
You could also check with your insurer if they've been with them for that long. EDIT - re-read the OP that the insurer is deceased but maybe he sold his business and records to somebody.
And look around for old insurance papers. Every once in a while, I stumble across something like that which had been stuck in a box for 20+ years. A couple years ago, at my parents house, I came across my wallet that I lost in 1989. It had a picture of an old girlfriend, my old drivers license and my old car insurance papers.
My current trophy find is a 1 Saudi Riyal note that my mother recently found in a letter I wrote as a 2nd Grader in Saudi Arabia and mailed to my grandfather in 1978. That bill with King Faisal on it, had been perfectly preserved for 45 years after my grandfather's passing in 1979. The letter was also pretty hilarious to read. My 7 y/o view of the world.
