Sometimes we get complacent, even when we know of the dangers
We tend to reason such wouldn't happen to us because we careful (yeah I was a victim of a clipboard malware).
Perhaps a keylogger that tracks and takes screenshot of everything you do. Perhaps a cookie hijacker that steals your cookies and allows you to access everything the other person does online. Who knows...
Highly unlikely Imo.
Quite a lot of addresses were drained which shows that he should have atleast five different seedphrase (considering HD wallet structures)
I doubt he would have entered them all within that two days.
I'm not used to wallets that deal with other cryptocurrency but from my experience wallets don't require you using your seedphrase to gain access to their app.
Likely a cryware which definitely should be through the meetings link. So far from what he shared that's the likely case.
I think they might sent him a file that contain malware, told him to open it. He trusted them
I expect he should have some level of paranoia towards opening sent files from unknown source.
Well he's same person that had 150 address holding approximately a million in a single device.
150 wallets and all on the same device? I would not be surprised if he was advertising himself as a privacy and security expert, what an idiot.

Not everybody listen to their own advice. Knowing the right thing is one thing but doing the right thing is another.