the moral of the story is to keep bugs you find in proprietary software secret or, since trying to alert the creator to the issue can get you in trouble, sell the secret to someone who will use it
you have no obligation to help proprietary software work and if you do discover a flaw, exploiting it for personal profit is not any different, morally, than using the software to begin with
the reason he was threatened is because fear of lawsuits on the part of the proprietary software vendor. This is the nature of proprietary software, making perverse incentives for your behavior
he was incentivized to sell the information, acting morally with regards to an immoral system was his error. It was his duty to exploit it, not report it