I’m not sure I would bet on the web itself still existing in 100 years. That being said, I think the biggest risk here is your hosting company going out of business, or changing their products such that they won’t host your site anymore.
If you’re OK with your website having a big ugly URL (which might not be a problem if you use a QR code to point to it anyway) then hosting a static website on AWS S3 might be your best bet. There’s so much money flowing into AWS right now, I imagine there will be enough interest to keep it going for several decades to come.
EDITED TO ADD As far as i know you can prepay your AWS bills, so you could prepay a massive amount and hope it outruns future price inflation
If he's hosting just some textfiles (i.e. a few KB), he won't have any aws bills that he would need to worry about prepaying. (I have a static s3 website, low traffic and low storage size, and i have not paid anything on it for years).
At least that's the case today, but the policy may change tomorrow. It's not easy to guarantee anything 10 years from now, let alone 100 years.
It seems the OSM ecosystem already has everything you need. Mapillary, OpenStreetCam or StreetComplete can be used to take geolocated photos that are then used to draw a map. The HOT (Humanitarian OpenStreetMap Team) has organised crowdsourced mapping efforts in Africa for many years now, and has developed open-source software to support those.
