Also known as "Russian refactoring".

Every self-respecting Russian programmer will always try and convice you to rewrite everything from scratch. Regardless of the context. I'm pretty sure it's in their DNA.

He can't do that, since he doesn't have the hundreds of man-years of domain knowledge that the authors of the code have accumulated. As a software developer, he probably doesn't know the first thing about how a chemical plant works. ("The program itself is a physical model of a complex chemical processing plant; the team that wrote it has incredibly deep domain knowledge but little or no formal training in programming fundamentals.")

Understanding 200K lines of complex code well enough to be able to rewrite it is hard enough, even if it's just pure database or graphics software. If it's about something you know nothing about you're going to have to learn something about the domain first, just to understand if the output you're seeing is anywhere close to correct. You're going to need to know whether some odd behavior of the code is essential to its correctness and must be preserved, or an accident of its particular implementation which can be changed or removed.

Also, you can't just dump some system that people depend on to get their work done. You have to maintain it and fix bugs while writing its replacement. Given that he's just a lone developer and doesn't have the budget or authority to hire a large team of professional hackers to help him, his only chance for success is to start learning about the system, adding tests and making small refactorings.

Sorry, but this is really bad advice. It is very tempting to redo something from scratch, but in doing so you will end up spending a very large amount of time fixing issues and adding details that were already done in the original code.