Yes, their prayer is valid. If they prostrate on the edge of the turban and feel the hardness of the ground, their prayer is permissible. It is narrated that the Prophet, peace be upon him, used to prostrate on the edge of his turban (as mentioned in Musannaf Abdul Razzaq 1: 400). It is also permissible if they prostrate on a turban that is not attached to them and feel the hardness of the ground, so it is the same if it is attached to them. However, it is disliked to prostrate on the edge of the turban without necessity due to heat, cold, or roughness of the ground. Refer to: Al-Bada'i 1: 210, and Al-Kanz 1: 117.