The genre simply developed.
In early 50s westerns like The Gunfighter or High Noon the hero loses the matter of course of his actions, some of his certainties, and thereby the genre itself began to lose its certainties. The hero questions himself or the reasons of his doing. The hero was now in a crisis and could become thereby tragic.
And he began to lose his psychic stability. In all of Anthony Mann’s westerns there is at least one scene in which the hero (often James Stewart) tries to tear apart one of his foes with bare hands and a manic expression on his face.
The hero is no longer entirely a mythological figure, his decisions were now also substantiated by reason and later by selfishness.
As I said before High Noon is not really a revisionist western like The Ox-Bow Incident was 8 years before, but it broadens the conventions of the genre. In the town tamer westerns (like the classic Dodge City , 1939) the hero is always fighting alone against the baddies for the future of a cowardish society. Will Kane also does what a man has to do, but he does it no longer with the approval of the society he fights for. Which leads to the fact that the end is not necessarily a promise for the future, but remains ambigious.
Lode, your friend should try to get a copy of Georg Seeßlen’s book Western Kino. The old rororo TB edition from 1979 (there’s also a newer Hardcover edition) is very good and gives a perfect overview of how the genre develops from early simplicity to the complexity of the twilight westerns and the baroque variations of the SWs.