Earlier quoted context omitted.
What's the goal? Does your grandma want to start a blog and you're talking about hosting the HTML from that folder, or do you want to be able to access the folder to read/write files, or something else?
The goal is to host a html page and/or share a file.
But to answer your question, you'll need to run a CLI daemon on your grandma's computer. Something like ngrok static files would probably be the easiest:
https://ngrok.com/docs#http-file-urls
But since you're already setting up one daemon in that case, I'd use Cloudflare Tunnel and also run a basic webserver or WebDAV server alongside it to give you more control over how the files are hosted.
Also pretty sure you have to pay for custom domains with ngrok.