Drayton Kedavra was a shady man. He did Dark dealings all the time and and he was semi-famous amongst the Dark community. Not a supporter of Blood Purity or the like, he just loved the Dark Arts.
By the time his Hogwarts letter had come, Salazar was proficient in the basics of the Dark Arts -
Scar on his chest (due to an accident with Dark Magic)
She mentioned looking for Emma Birch and Salazar had heard that she was a ghost of Hogwarts - a Slytherin student too. She currently haunted the dungeons and Salazar had seen a glimpse of her once.