I'm not going to claim to know anything about anything, but is it possible that Martin loaned the guitar so that copies could be made? Remember when Grot's guitars were loaned out when Guild was getting the American factory going again? Maybe they had no clue that the genuine guitar was going to be put on set in a scene. Maybe the director did it intentionally for the "effect". I watched the video of the scene, and in all honesty I don't think the actress' reaction was believable for the scene that was supposed to be happening. It was believable for her knowing an irreplaceable guitar was just smashed, but she is obviously looking around the set when she realizes what is going on. I cannot imagine looking anywhere other than where my prized guitar lies in splinters. I would definitely not be looking about the room yelling, "whoa". But that is just my opinion and worth every penny you have paid for it.
Side note, I did not know until reading the reports that this director did Pulp Fiction. I rented it back when people were raving over it. I couldn't get past the first 10 or 15 minutes. The language was disgusting and the subject matter deplorable. I was not going to sit through another tortuous moment to get to whatever the good part was that had people so excited. Decidedly not my definition of entertainment. If I missed a "really great movie" so be it. I have managed just fine in the meantime.
So, who is at fault in the guitar smashing incident? We will likely never really know. There is probably plenty of blame and finger pointing to go around but none of it will undo what has been done to that guitar. My opinion, based on not knowing anything about anything, is the director was in on it because his movie budget is so huge that smashing an historical instrument can surely be appropriately compensated without even a blink of an eye, and he gets a "genuine" reaction along with a lot of free publicity. Pretty low in my opinion, but what do I know?