There is a pizza shop in town run by a Jew who is not Shomer Shabbat. He claims that he is using only Kosher ingredients. May we rely on his word?
Somebody who is not Shomer Shabbos, i.e., he does not live as an orthodox Jew, is not believed for all matters of religious practice. One may only eat in the shop if there is a religious and trusted Jew who supervises the ingredients and the production.
Sources: See Chulin 5; Shulchan Aruch, Yoreh De’ah 119:7, Rashba, no. 430; Bechoros 29b and Shulchan Aruch, Yoreh De’ah 311:1; Pri To’ar, Yoreh De’ah 1:3 and 119:; Rivash no. 500; and many other sources.