When you see this error, it's when trying to download the update via WiFi I assume?
Do you think he could follow those instructions for updating with a computer?
More or less just needs to make sure the Mac is up to date. Then plug in the phone and see if it shows up in iTunes (or Finder if running Catalina or Big Sur)
If you were to help, you would need direct access to the MacBook. (or any computer capable of running iTunes).
How did you get the phone to him? Could you get the phone back and use a computer of your own to do the update?
I suppose you could also try using software like TeamViewer to gain remote viewing of his computer, but it would involve him installing the program and running it.