She recognized him from the start.
She had seen his picture in the TARDIS. Or rather, the TARDIS had shown him to her. Shown her the man that had come before her husband. Shown her the reason her husband was the way he was. Shown her the Oncoming Storm, the One who had lived for far too long, the one who had lost far too much.
With her normal "come hither" smile, she talked to him. The redhead was with him. Donna.
Oh, poor, poor Donna. Only after a lot of coaxing and threatening had her husband told her the story of Donna. Donna, the "most important woman in the entire universe", was the companion who ended with the worst fate. Most of his previous companions died, were lost to unreachable places, or left. But Donna forgot. She forgot everything she had seen, everything that had affected her and changed her. She lost all of the growth she had gained during her far-too-brief encounter with him. Transforming from just Donna to the Doctor-Donna was one of the most important events in history (albeit barely anyone knew it had happened). But the Doctor-Donna had to be kept hidden from herself, lest she burned.
Becoming confused by his behavior, she finally asks.
He doesn't know her yet.
Their meetings had always had a predictable pattern to them, although it didn't seem so at first. As she grew older, he grew younger-or it seemed that way with time travel and all that. For a few times, they were both nearly in the middle. It was those visits that she looked back on with the most love. Finally meeting in the middle, he had said. But soon she grew older and knew him more, while he became younger and seemed to know her less. But if he didn't know her at all, then that basically meant one thing:
This was the last time she would ever see him.