During Patty Jenkins’ Reddit AMA, a fascinating interpretation was brought up regarding this scene:
“My interpretation of the revisit was that it actually wasn’t necessarily what he said. She (and the audience) has no way of knowing what he said. But the whole lesson of the film is about faith. Believing in something for the sake of hope. So she thinks back to that moment and chooses to believe that he said something beautiful and moving. Because that’s what she needs to believe in order to have hope.” – Leagle_Eagl