Boxing

[wenshu]

Meanwhile, he maintained that the "only legal stance in boxing" was a side-facing, narrow, lead leg stance, "like in fencing."

Switching stances can be frowned upon by coaches because unless you know what you are doing you end up exposed with your legs crossed which is a big mistake and a seasoned opponent will capitalize on it. Maybe your friend is misinterpreting admonishments not to do that from his coach as "rules".

but most boxers have their upper body square to their opponent

um, what?

The only time a boxer's upper body is square to his opponent's is when he is about to get KTFO.