When you use the fusebox trick, you only attach the hot (red) cable to the terminal in the fusebox provided for this purpose. The black cable (ground) gets connected to the door striker, which is attached to the car body and is therefore grounded.
Modern Porsches kill their batteries after just a few weeks of sitting. Tell your friend to get one of these:
Amazon.com: Battery Tender 021-0123 Battery Tender Junior 12V Battery Charger: Automotive