Let's assume that this project merely clones the game mechanics and uses scans of its artwork, and all it does is providing a software version of the game, available at no cost.
With those assumptions in mind, would it be OK from a legal/copyright POV to release such an adaptation to the general public?
No, he doesn't. Any of those alternatives you mention (apprenticeship, etc.) require someone senior to you overseeing your tasks/progress. That someone can be a colleague, boss, CS teacher, etc. It's up to the entity overseeing your development (be it a school, or a company) to provide such expertise.
Once again, it's OK that you (person, programmer) don't know that. But as a company, there should be a strong guarantee that you (company) have the knowledge to build a good product. It's fine for a company to have some people who don't know this stuff, but not to not have anyone capable (and in a position that allows them to) notice these kind of mistakes.
This indeed should not be news. It's OK for you as a developer (even a professional one) to not know about this. You can learn it from your colleagues, debugging poor performance situations like this, etc.
But it's not OK for a company to offer a service built in such a naive way (naive from a comp-sci point of view). To me this kind of mistakes are a symptom of having a poor CTO, poor hiring/dev practices, etc.
With those assumptions in mind, would it be OK from a legal/copyright POV to release such an adaptation to the general public?