If they did, Microsoft sues, and the programmer is asked under oath, everyone knows. Unless the programmer decides to lie under oath, which will only make matters worse eventually.
If they claim they just reverse engineered the code, they're in an equal amount of trouble. Reverse engineering is illegal in many jurisdictions, unless you manage to do it in a very specific way which should still results in different code than what Microsoft wrote.
The more source code leaks, the harder it becomes to claim plausible deniability. Ethically, I think that's wrong and should be fixed, but the copyright and software protection law is against compatibility projects like these when it comes to legislation.
If they claim they just reverse engineered the code, they're in an equal amount of trouble. Reverse engineering is illegal in many jurisdictions, unless you manage to do it in a very specific way which should still results in different code than what Microsoft wrote.
The more source code leaks, the harder it becomes to claim plausible deniability. Ethically, I think that's wrong and should be fixed, but the copyright and software protection law is against compatibility projects like these when it comes to legislation.