This feels like an expensive fix to a problem that seems worth digging into. Is the hosting provider the one not sending the header? Or is it Cloudflare or the web server? Saving 60 dollars a year to find that out seems worth it to me.
I have been hosting my minimal static website on GH Pages for a long time (and I've used it for testing many static websites) for free.
And I've been browsing blogs/websites hosted on it with no issues.
It's probably her host using a wonky server.