Yes, her prayer is valid. If she prostrated on the edge of the handkerchief and felt the firmness of the ground, her prayer is permissible; as it was narrated that the Prophet, peace be upon him, "used to prostrate on the folds of his turban"; in the Musannaf of Abdul Razzaq 1: 400, but it is disliked without necessity. See: Al-Bada'i 1: 210, and Al-Kanz 1: 117.