Ask questions and do your own research. Even though you know the person who had the car before they traded it in. I would do a carfax to get an idea how long they have had the car on their lot and whether it was traded around from one lot to another. As you know the person who had it before this dealership talk to the guy and ask him if it had any issues he noticed.
Physically look at the car, drive it and get a feel for how it drives. Look at the body panels, is the gap between say the hood, trunk, etc and body panels the same along each seam? Open the doors, look the the door sills; is it still the original factory color? Maybe bring a magnet with you, if their is any body damage on the car, anything that should be metal should still be metal so if the magnet doesn't stick to those areas perhaps bondo has been used in that area. Remember you are looking for anything that you can use to your advantage to drive the price down abit.
Talk with the salesman, if you know they took a 19.5K hit on it chances of them going below that is slim to none, but you if mention any of the issues you found during your inspection (but don't mention you know the guy who had it before them), mention you saw the car has been sitting on their lot for X amount of time and that you seen a similar car on another lot for X cheaper but with more miles and they were willing to throw in brakes/rotors. Basically try and haggle with them some, either trying to get the price you want or get close but with basic maintenance done to the car.