If they aren’t going to share it what’s the point of writing it on the site? They could write it as a draft, perhaps?
Have you got a budget? I can imagine a wide range of solutions if there’s a budget, but without knowing the budget it’s hard to know what to suggest.
If they were to write in markdown (and if they aren’t, discourse is a terrible tool), a plugin or some markdown editor could convert it to pdf. Some editors might also be able to pull the latest version from the raw post endpoint. Or maybe a clever github action could pull all of the posts on a topic (maybe even a private topic) into files that could then become a pdf or static site.