Law enforcement can use a confiscated cell phone resulting from an arrest of your friend for prostitution to make further contact with her "clients" as part of the investigation process of the crime and to make further arrests.The cell phone was the "fruit" of the arrest and could be used to make further arrests. The "clients' of your friend have no justifiable expectation that the person who was texting them post your friend's arrest was not law enforcement.The "clients" simply could have ignored the mesages and not gotten arrested. I suggest that your friend who was arrested for prostitution contact a criminal defense attorney to assist her with the pending charges against her.