Is it possible that your friend had an induction coil under the driveway? I'm not a scientist, but there is no identifiable source of power being harvested sufficient for what was observed to my limited understanding.
That's why I am intrigued by LENR...I can see where the power comes from...it's unzipping what was crammed into an atom by a supernova for the metal, or the big bang for the hydrogen...I'm sure there are lots of yet unknown ways to release that.
In the case of your friends device...and the only thing worse than my physics is my electronics, but when I see coils and capacitors, I think LC tank circuits...then I think fields. Since the earth's field has been found to be far too weak for kinds of outputs you were observing, I begin to wonder about other field generators that could be being harvested by the apparatus...like a buried coil. Just a theory that's worth what it costs.
That said, an NDA, and a qualified reverse electrical engineer might prove equally useful.
Best of luck.