imported from Australia a while back
How long ago?
By "imported" do you mean it was cleared through US Customs? Or that it was brought into the USA by unknown means?
The guy doesn’t have the papers
Which papers does he not have? Oregon Title? Title from another State? Import documents from US Customs?
If the seller literally has nothing, how do you even know it's his car to sell?
If he has a Title, things should be easy. If he can't find the Title, but knows he had one at some time in the past, then the State of Oregon (or wherever it was Titled) should be able to issue a duplicate. If he never had a Title, you'll need to apply for one.
If the seller doesn't have any of the Import documents, but knows the car was cleared through US Customs, then you should be able to get a duplicate of the Entry Summary from your local Customs office.
If the seller doesn't have any of the import documents, and the car was never cleared through US Customs, you should be able to visit your nearest Customs office and get it cleared. You'll need to declare that it's over 25 years old and that the the engine and emissions systems are unmodified (form HS-7 and 3520). You'll also need to pay duty on the declared value (usually the sale price). After doing that, they should issue you an Entry Summary (or Proof of Entry), usually a form 7501. Once you have that, you should be able to go to your local DMV and apply for Title.