It's the thickness of the new bracelet, which is noticeably thicker in profile closer to the watch than the previous mesh and tapers down towards the end. If he's wearing it high up on the bracelet where it's a few milimeters thicker, it would make a lot of sense that the clasp would have some trouble closing.
Yea, I’ve got 6.75” wrists so I wear pretty high up on the notches. Likely a quality issue. Here’re some pics of the clasp. It closes just fine and would be fine when wearing.
You can see that the locking pin doesn’t close all the way even when pressing in it.
But the clasp still locks properly: