Normally, taking a few pounds off a bow, particularly one that the bowyer made himself, would not be considered a high risk operation. However, $hit happens I guess. Using a medical analogy, if a patient dies during an operation normally considered to be low risk, the doctor is not liable unless he was negligent. Who knows if your bowyer was negligent or not? Nobody was there watching him. I would guess that while the bow was on his tillering jig, it just broke.
Now you have had two bows made by the same bowyer that have had defects. If other people have similar experiences, he probably won't be in business much longer. If I were the bowyer, I would try a little harder to make you happy than he seems to be doing.