You can buy several books about Khan's photography: http://www.amazon.com/Dawn-Color-Photograph-Albert-Archives/dp/0691139075/
This was from Albert Kahn's photographer Stefan Passe. I have the book The Dawn of Color Photography at home, but I don't think it gave the reason for her execution. As to why he didn't intervene: he was simply documenting reality in Mongolia. That and what astute_stoat said.