Sarah was a human that Bernice Summerfield met on Crex.
After Guy de Carnac's death, the Seventh Doctor took Bernice to a market on the planet Crex. While he went to meet Laylock to obtain the technology to make himself temporarily human, Benny found a beer tent and shared drinks with three human women she found there.
One of them, Sarah, was on Crex for reasons which Benny never learned. (PROSE: Human Nature)