Robiya Otamurodova told RFE/RL's Uzbek Service that he visited Turgunov earlier this week in a prison in the western city of Nukus and that a large section of Turgunov's back had been burned.
Turgunov was arrested on extortion charges on July 11. He denies any wrongdoing.
Turgunov is the executive director of the Tashkent-based human rights group Mazlum and also a member of the unregistered Erk opposition party. He was arrested in the western Karakalpakstan region, where he was acting as a lawyer in a civil case.
Turgunov's colleagues say his arrest is politically motivated and aimed at silencing the activist.
In November 2007, the UN Committee Against Torture concluded that torture and ill-treatment remain "widespread" in Uzbekistan, after scrutinizing the country's torture record during its periodic review of its compliance with the UN Convention Against Torture.
New York-based Human Rights Watch also concluded that torture is "endemic" in Uzbekistan's criminal justice system.
Turgunov himself told RFE/RL's Uzbek Service last November that the situation in Uzbekistan had worsened since a visit in late 2002 by former UN special rapporteur on torture Theo van Boven.
"It is clear that torture is an endemic problem in Uzbekistan," van Boven wrote in 2005, "so much so that I concluded it constituted a systematic practice in the country. I found the numerous accounts of torture I gathered from victims and their relatives so consistent in their gruesome description of torture techniques and the places and circumstances in which the abuse was perpetrated that there was no way to deny the pervasive and persistent nature of torture throughout the investigative process."