Yeah this seems like one of the easiest access models to create an excellent security model for -- basically just static publishing. Sounds like they were in need of a rewrite anyway!
It probably seems daunting but to be honest this feels like a weekend's work at this point with LLM assistance. Not to be glib!
But it also destroys the business model behind the site.
Sure, you could technically redesign to handle the bot traffic, but if the bot traffic is just taking the data and reducing any need for humans to visit the site, why is he putting in the effort to maintain the site?