Don’t blame him a bit! Would do the same thing.
He should have a PI check to see if any of them were working full time for two companies (i.e., working for Twitter, plus also working some other company that is equally naïve).
Back in the mid-80s I worked for Burroughs Computer Corporation, they hired a salesman from IBM and paid him over 100k/year which was a small fortune in the 1980s.
He stayed for about 1 year, supposedly was injured and went on full disability, when that ran out and came back to work long enough to supposedly again get hurt and start drawing full disability, during this 2nd disability stint, a competitor called wanting to verify past employment.
Come to find out, the former IBM salesman and his next door neighbor who was a Doctor and certifying him as disabled were running a disability scam, splitting the 100k he was making from Burroughs until it ran out and attempting to scam the next company that would hire him.
Ghost employees do exist in many forms.
I would also be looking for fake vendors that are getting paid for services that never existed.
Due diligence is racist and patriarchal!