And in this scenario, it might not even have been an assessment of risk, but rather "just to mess with him". The reporting person may have thought it would be a funny prank to have him questioned by police over the message and that would be the end of it.
It doesn't seem so likely that a human in the airport happened to read his message, or that surveillance cameras at that airport OCR all peoples' phones (although I wouldn't be completely surprised if they did)
That doesn't seem at all likely.
Occam's Razor prefers explanations constructed with the smallest possible set of elements.
In this case, that would be someone in the airport reading his screen and turning him in, or someone in his private chat turning him in.