So if you write some toy PID controller for your kids LEGO project and post it to stack overflow, should you be held liable for all the places it is used?
If this person presented it as a real solution, however, then yes I believe he should be held liable, as much as a lawyer would be for recklessly giving out legal advice.
In either case, The developer who built upon the solution without validating it or verifying its correctness should bear most of the blame, regardless of how the answer was given.