The frame is begging for something stiff that can plow, and the bike is light enough to afford the extra grams but running such a big fork at 140mm could be overkill for sure. It can only improve the way he rides, but I concede a Pike/Lyrik or Fox 36 would also do a superb job, and if you could find something in that vein second hand on RB it would be worthy.
Personally, I just love coil forks ever since the old Z1 and if I was upgrading would chuck a Mezzer on the Bronson in a jiffy. The old 130mm Sherman I had was a great fork, I can only imagine a 160 Mezzer would be like that but betterer.
The Cairns terrain has plenty of chunky stuff.