What happened to Sal after Phoebe became suspicious of Ms. Cadaver?

Phoebe Winterbottom's still deeply traumatized by the sudden disappearance of her mother. Inspired by a Longfellow story read out loud in English class by Mr. Birkway, she gets it into her head that Margaret Cadaver and her strange mother Mrs. Partridge are somehow responsible. So she and Sal start snooping around Margaret's place, looking for clues that will point to Mrs. Winterbottom's whereabouts. As well as finding some suspicious items, the girls are shocked to find Mrs. Partridge reading in the dark. She also creeps out Phoebe by telling her that she's seen her brother, even though Phoebe doesn't actually have one.
After her and Sal's little escapade, Phoebe is now more certain than ever that her mother's been murdered. Sal, however, is not so sure. Drawing on her own personal experience she tells Phoebe that perhaps her mother left because she wanted to leave, but for some reason couldn't tell Phoebe about why she was going. But Phoebe is having none of it. The very idea that her mother would just take off like that without a word is just too horrible to contemplate. So she lashes out at Sal and silences her at once.