Think the car belonged to a German couple which get divorced. Though the car came from Germany (not sure!), but stolen car..... Honestly, I don't think so. Getting new plates (Spanish) from German one's, plus change them to historic vehicle is a lot of paperwork. Also, when the vehicle was first imported to EU you must fill many many papers plus pass through an accurate inspection, it doesn't matter in Germany, Spain, etc...
So no, I'm not thinking this could be a stolen car.. I'll keep investigating.!
Also, I don't think my friend really cares about the history of the car nor the state of it, concours, originality, etc.. This car has many aftermarket features, plus some originals as well..
Josep