Why add another layer? Right now the only tool he needs is an editor. That's it. Can't get any simpler than that. He also probably has a set of templates by now.
At the end of the day the drudgery they are doing is writing the post, something they'd have to do anyway on a static site generator. Writing the raw html likely takes no friction at all, since they are probably copying and pasting some template or at this point know all the tags they use by heart so its no harder than a more abstract method.
Comparing the complexity of operating and maintaining a SSG to using a simple text editor is just about the most HN comment I've ever read. My blog is written with Gatsby (which I use as a SSG), and about once per year I have to dedicate and entire weekend to troubleshoot because I can no longer run the SSG due to changes in the Node/npm ecosystem. There is absolutely no corollary to using a text editor. Literally nobody will have to spend a weekend figuring out "what do I have to do to get notepad to open a .html file with today's technology". Nobody! Notepad just opens the .html file, just like it opened the same file a year ago, or 10 years ago. No need to burn weekends over opening a file!
You can drop in whatever text editor you like though. It's only a dependency like saying a keyboard is a dependency to writing code. There will always be new editors or you can use a 40 year old one just fine too. Can't say the same about the static site generator though, maybe your setup works when you swap one generator for another maybe it doesn't.
That seems like it'd do the same thing and just take out some of the drudgery.