For my $0.02, if he hasn't tried running an Ethernet cable from the fiber modem to a switch and then a cable p, or two, to his computer(s) for the zoom calls, and streaming, I think that he should. That would rule out a bunch of things, and should be step one.
I have no particular reason to think that his issues aren't just fiber overloading, but there are lots of ways to mess up a WiFi network, even more to mess up a mesh network (combined 2.4/5GHz SSIDs, some LED lights, dimmers, garage door openers, poor mesh layout, old IOT devices, etc.)
He might want to look at WiFiman, from Ubiquiti.
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I wouldn't bother running a cable through the woods. There is lots of publicly available data out there that demonstrates thatStarlink works for multiple concurrent zoom calls and streaming. (To which I can certainly attest.) If he doesn't believe what is publicly known, then I think it has reached the "you can lead a horse to water, but you can't make him drink" stage, and I would leave him to it until he is willing to listen to your advice and follow it. Training horses has taught me a lot about how to do deal with problem cases gently. Just sayin'...
There are networking consultants out in internet land that would be more than happy to consult for your neighbor.
All the best,
Peter