Sorry...swamped here with the usual, super-late, Christmas Rush. It never fails.
I suspect that what your friend did very simply was NOT have an ncx. Many people who work strictly in Word don't. They use anchor tags (bookmarks) in their Word files, and when they build the file using Kindlegen (via the KDP, in other words), the software "builds" the Guide items for them. So, in other words, in a Word file, your friend put an anchor (bookmark) called "TOC" before his typed TOC. He put an anchor called "start" wherever he wanted to start (n.b.: the latest KG has a bug, and will not, for whatever reason, start at the first html page after the cover.).
That's why his works. I know, I know--it's crazy-making. For a while there was something else, I forget what the hell it was, that worked better in Word than it did in html, making mobi's. Amazon makes certain assumptions, when it's writing its software, about what noobs will do with Word, and compensates for it. Like what they did for your friend's book.
What I don't know, because I have not run into it, is how to circumvent what you're experiencing. I don't know how to get the toc that's being displayed to show the entire thing, so that someone on, say, chapter 65 can leap back to Chapter 1, without having to go "Beginning," and then "TOC." I don't. I'll have to think about it.
Sorry I can't help you more, right this second, but that's why your friend's book works as it does and yours does not--he doesn't have an NCX. He used anchors and Kindlegen built the Guide for him.