Didek was born on 11 June 1910 and died on 27 October 1975 in Ljubljana. He studied painting from 1928 to 1933 at the Zagreb Academy of Fine Arts (under Profs V. Becić, Lj. Babić, T. Krizman). Before the war, he was employed as a teacher at grammar schools in Krk, Zagreb and Sarajevo. In 1946 he became a lecturer at the newly founded School of Arts and Crafts in Ljubljana, where he started art theory classes. From 1956 to 1967 he taught painting and art theory at the Ljubljana Teaching Academy, and then from 1967 to his retirement in 1974 he was Full Professor (Small and Large Nude classes) at the Ljubljana Academy of Fine Arts. In 1975 he received the Prešeren Award for his life's work.

In his early period, when he was a member of the Neodvisni group, he painted landscapes, portraits and still-lifes in the "Colour Realism" style, whereas after the war, his explorations took him as far as geometrical abstraction. Under his guidance, some participants of the first Slovene art colony "Izlake-Zagorje" (an event started in 1964) focused on the (post)cubist style of painting, which is a rare phenomenon in Slovene art.

Apart from being a painter and graphic artist, he was an illustrator and designer (book and graphic design, design in pottery and gold). He also designed monuments and furniture for several industrial plants and schools (sometimes in cooperation with his brother and architect, Josip Didek). As an adviser he participated in the making of short films about different artists and art techniques. Since 1990 a large portion of his work has been kept in the permanent collection of the Božidar Jakac Gallery in Kostanjevica na Krki.