The options available to the author are:
1. Manually add a link to every page on the site each time a new one is added.
2. Give up on hand-writing HTML and use a server-side templating system
3. Give up on having navigation on each page
4. Load the navigation through JavaScript
Given that they want to hand-write HTML, and they (presumably) want it to be easy enough for everyone to do, the JS options seems like a reasonable choice.