Yep worst case scenario would be they reposess the car from you and then you'd have to go through the courts to attempt to get anything back from the guy who sold the car to you which may be 'not a sausage' in the end.
When you do a HPI check it will say who the outstanding finance is with. Once you have the details including the loan ref number go to the bank and get a bankers draft made out to the finance company and pay the remainder to the owner. This is the only way that you can be sure the finance company will receive their money and don't come after the car.
It's worth the extra little bit of effort for the piece of mind.
This is one of the main reasons to do a HPI check, if you ignore the information and just pay the asking price in cash you might be allright but you might get stung and end up without a car or money.
If the owner is going to pay the finance company anyway then he should have no problem with this.