I too shared the surprise of the author when reading about acts contra bonos mores as interpreted by the BGH in the “Inaccurate audit” case. Just like the author I felt it was too broadly interpreted and stretches the law too far.
Negligence is typically not a state of mind one would perceive as going against good morals and custom. It is a state of utter carelessness, but not one that contravenes what society believes to be ethically valuable. The BGH circumvented this issue by judging the defendant acted willfully. But the facts of the case paint a totally different image. He seemed not to care at all either in the performance of his duties or in the consequences they would have. But labeling it contra bonos mores seems to me quite a leap. Surely such acts are reprehensible but not so far as to call them “acts against the good morals.”
There are two risks, in my humble opinion, in using boni mores to impose tortious liability in such a broad manner.
First, it can be perceived as outcome-based justice. The court wanted to find him liable and extended an area of the law that was relatively loosely defined to do so. One could understand this as judicial activism.
Second, I believe it does harm to the notion of boni mores. It effectively blurs the line of what is against good morals and values and what is not. This seemingly all-inclusive category has in a way become a filler for an otherwise partially ineffective tort law. Arguably one could say the same thing about the tort of negligence in English law. But the difference lies in the fact the tort of negligence was incremental progress of already existing law; it was new but not radically new. The application of boni mores in this specific case, on the other hand, constitutes an unsound expansion of a morally charged doctrine.