eat pray footnote

My wife has been reading the bestselling eat pray love by Elizabeth Gilbert. I was glancing over her shoulder as she perused the book on our recent flight home from Moncton, New Brunswick (via Toronto, Winnipeg, and Calgary) and noticed the following quote from page 208:

In 1954, Pope Pius XI, of all people, sent some Vatican delegates on a trip to Libya with these written instructions: Do NOT think that you are going among Infidels. Muslims attain salvation, too. The ways of Providence are infinite.

Regrettably, Ms Gilbert does not provide a footnote citing her source for this fascinating quotation. I have searched high and low for anything like it and have come up empty.

Pope Pius XI was pope from 1929 to 1939. During that time, he took a pretty standard position on the potential for salvation outside of the Roman Catholic Church:

If any man does not enter the Church, or if any man departs from it, he is far from the hope of life and salvation." - Mortalium Animos," PTC:873

If Pius made the alleged statement in 1954, he was speaking ex cathedra indeed!