I think she’s the one in the photo (photoshopped) showing her, Carla Bruni, and Mooch going up the stairs - from behind. I think she is wearing a red dress, Carla is wearing a blue dress and Mooch is in a white top and pink skirt.
Yes, Princess Letizia is the 3rd woman in that picture (that’s sooooo cruel toward MO by comparison to the other two). Love that pic.