This is one side of the story. There's likely more than just this text message that raised suspicion. But the law is quite clear that anyone applying for admission to the US is presumed to be an intending immigrant and the burden is on them to prove they will leave before their duration of status expires. Literally millions of people per year are denied visitor visas because they can't meet that burden of proof. Not…
How can you prove something that didn't happen yet? Best you can do is provide clues and circumstancial evidence but no proof.
It's good that they were being so vigilant! Let's not forget that USA has so much trouble with UK citizens overstaying their visas. /s