In 2007, Kathy and Sally visited the abandoned house Wester Drumlins, investigating some mysterious writing on the wall which was left by The Doctor in 1969. The writing was a message intended for Sally. While they were investigating, a man came to the house with a letter for Sally. He claimed to be the grandson of Katherine Costello Wainright née Nightingale. At that moment, Kathy was approached and touched by a Weeping Angel. The angel's touch displaced her from her own time and she found herself in Hull in 1920.
When Sally read the letter that Kathy's grandson had left her, it revealed the truth. The letter was indeed from Kathy and she explained that she had settled down, had married and had children. It also explained that she could not see Sally ever again as doing so could damage the timeline and the universe. She had been contacted by the Doctor in 1969 and told this information then.
Kathy lived a full life and died at roughly 90 years of age.