The VO people send a sample of their engine oil to Blackstone for oil analysis (
http://www.blackstone-labs.com/index.html). If he's concerned, he could send a sample, tell them what he's worried about and see what they say.
Did the current owner say how he's running the VO? Two-tank or one? Does he know how well its heated or know the fuel temp?
Its a big leap of faith to buy a car that's run on VO. I'd do it if I knew the previous owner well enough to know his VO habits/use. Otherwise, I'd be wary. Unless you don't need the car to run for years and years.