I always ask it to create some documentation.

Reverse engineer it into scratch? (or similar block-based beginner-friendly language, with comments)

Maybe the AI could capture video of program runs and then show the user representative small clips from these videos that depict distinct features?

Visual programming

I'm having the LLM work with human-readable behaviors for each component. You can have the LLM take those behaviors and create a Mermaid diagram, for instance.

You can’t, because the reason the code exists is because you need its formality in order to eliminate ambiguity. Everytime code is explained without showing the code, there’s a baseline of understanding that’s required to filter the additional ambiguity.

The only approach left is to refer to the domain. As the person is familiar with it, you illustrate the behavior that is currently programmed by the system. The program itself should be a blackbox. Only knobs and sliders should be left (aka any professional software).