This is the sentence in question:
To research the book, See traveled to a remote area of China that, she was told only one other foreigner ever to visit before her had ever visited.
I understand how to use parenthetical commas and I think this is an example of using those commas. I feel quite confident that there should be a comma before "only" so that it would read..."remote area of China that, she was told, only one other foreigner"
Edit: comma before only