Can a police officer give you a DWI if he never saw you driving and you are sitting in your car on private property?

You can indeed be arrested for drunk driving even if you were not driving at the time.  Actually an officer need not actually observe someone behind the wheel of a moving car or other vehicle.  All that is necessary is evidence of driving. The fact that your friend was alone could be such proof (how else did he get to the property; and he had to use public roads to get there). Additionally, I don't know what he may have told the officer at the scene.  That together with a failed sobriety/breathalyzer test (you did not indicate) would further establish that he not only exerted control over the car but did so at a time when he was legally intoxicated.  Therefore it can be argued that the elements of DUI charge were met.

However, your friend could still fight this charge.  There are numerous defenses that could be put forth in his defense that might result in a reduced or possibly dismissed charge.  At this point his best bet is to consult with a DUI attorney in the area where all of this occurred.  In addition to any legal arguments that can be made on his behalf, they will also have contacts within the local  court that they can utilize to yout friend's best advantage.  A DUI is both a civil and criminal matter.  He really should have an experienced DUI attorney on his side.