I would propose a different path to try to figure out what the sound SHOULD be without all the intervening programs/internet issues. Simply set up his microphone, and using something like Reaper or Audacity, or whatever recording program he has, determine if the sound as he would be recording it is acceptable. First and foremost, the source must be good, otherwise all is lost.
If the recording isn't up to par, then all the fixes in the world won't make it better. If the recording IS good, clear, and accurate, then you can start playing with the other programs, sending the GOOD audio downstream and tweak as needed.
Also, FWIW, I don't think you have ever stated which video conferencing software is being used. Is it Skype, Zoom, Google Meet, MS Teams, Citrix? A combination of programs? You can't fix problems if you don't understand the sources.