Wow! What gorgeous, gorgeous lace!
Since the meeting at Castel Gandolfo was private, I could see the head covering being optional.
There are photos of Laura Bush accompanying President Bush at a private audience with John Paul II at Castel Gandolfo wearing a black mantilla, as well as Nancy Reagan looking elegant, as usual, in a black outfit with a black mantilla at an audience with JPII while accompanying President Reagan.
I don't think Queen Sofia would make such a faux pas, but one never knows.