Where is management in this story?
Engineer A's acts seem not only unethical, but outlandishly so. As I understand it, A has replaced all of the drawings with his own, some of which are largely based on B's prior work. What was A thinkin g of? How did he expect management to receive this strange behavior? This account seems to suggest that management did NOTHING in response to it, which suggests management was either not paying attention, or being inexplicably reckless (even assuming that they do not recognize the design error, the seriousnes of which is not discussed).
In general, one would expect that what B should do would depend on what avenues for solving problems are afforded by the company, but if I am right about what this case implies about management– that management is behaving in inexplicable ways– that suggests that B might consider finding saner company with which to work.