I know that the piece that Jennifer Wells is playing when she is hit over the head is Chopin's Etude #13. But I believe she is playing a different piece when John Cassavetes first enters the room.
If it's what I think it is, it's part of the same piece that he plays when he's approaching Williamson Ranch in the fake nighttime car scene: Brahms Piano Concerto Number 2 in Bb.
Love those early TV night scenes - put a dark filter on the camera, shoot in sunshine (so it looks like moonshine), turn on the headlights and play chirping cricket sounds LOL