In 1943, Frieder collected testimony from an escapee from Treblinka extermination camp, which was also sent to the Working Group's contacts in Switzerland. Soon after the German invasion of Slovakia concurrent with the Slovak National Uprising in August 1944, Frieder was imprisoned in Bratislava. Released by the end of the month, he was arrested in the 28 September roundup in Bratislava and was imprisoned at Sereď concentration camp. Meanwhile, his family had fled to Banská Bystrica, the center of the uprising. Frieder's wife and daughter were killed in a German attack on the nearby village of Staré Hory, and his son was wounded. Frieder managed to avoid deportation from Sereď and survived the war.