This unnamed lady is a waitress from the Princeton University cafeteria whom Alfred Hofstadter (Leonard's father) had an affair with and cheating on his wife Beverly. This resulted in the two of them getting divorced. Beverly left him on September 22 which Sheldon knew all along and Leonard only found out in December, presumably on the 14th ("The Maternal Congruence").
She is indirectly mentioned again in "The Convergence Convergence" when Beverly reminded Leonard, who actually knew, that his parents' marriage ended because his father had an affair.