This is a terrible solution. You should’ve have to lose all of your content and settings just to get your phone to work.
Wrong. You won't "lose all of your content." The clean install is the best way to isolate the problem. If a clean install fixes the issue, you likely have ruled out a hardware defect -- and now you can focus on the software, in particular apps.
By the way, if he takes it to the Genius Bar or an authorized third-party retailer, and they can't immediately troubleshoot the problem, guess what they are going to do: they will wipe his iPhone and do a clean 10.14.1 install, start up the iPhone, and test Face ID. So it's going to happen anyway.
What's more, the best way to update older Apple products (iPhone, iPad, Mac) is with a true clean install with every major software update/release. You eliminate the detritus left over by all the incremental updates in iOS or OSX. You also clear out detritus from old apps. Apple products work as new when a truly clean install is performed. (And this was especially true with the Mojave updgrade and moving from +Journaled to APFS.)
So no, it's not a terrible solution. No, you won't "lose" all of your content. No, you won't "lose" all of your settings -- if you have proper data and password backups. Contrary to what you say, a clean install can greatly improve device performance and longevity,
especially in a networked environment (which I run). In any event, I don't care if you don't believe me. I manage an entire office of OSX and iOS products, so I have direct knowledge and experience. To say this solution is "terrible" is, in a word, ignorant.