He didn't get rejected for a visa. He got rejected for an ESTA. Travel on visa waiver is explicitly allowed for business meetings, as long as you're not travelling for "productive work" (meetings where you receive instructions are fine).
Business stuff is not at all a grey area. From the ESTA application website, appropriate use includes "You plan to travel to the United States for business or pleasure."
Meetings are fine. You'll note the visa he was approved for is a tourist visa. Which is also fine for the same use - given it was approved with the use of an invitation letter from his employer.
You need to say something very dumb to a border guard to get denied on the basis of exceeding the allowed use of the visa waiver, but if you do, you'd get turned away then and there, not rejected at the airport the next time.
"Something" certainly got triggered, but it's highly unlikely to have been about going for business meetings.
My guess would be that something got messed up with his previous departure, such as e.g. failing to register that he left on time, as that is one of the things that would pretty much instantly disqualify you from future visa free travel. I almost got into trouble for that myself with visa waivers (though pre-ESTA). The main reason I wasn't denied re-entry was that I was still within the 90 day window of the previous entry, so I provably hadn't overstayed, and they could see I had about a dozen or so short stays that made it very obvious it was a mistake. But that was in person, with someone manually looking through my passport stamps - I'm not at all sure it'd have gone as well today with automated pre-flight checks.
