Is one permitted to eat, after Shabbos, food purchased on Shabbos by a non-religious family member? Is it necessary to wait the amount of time it took to buy the food?
One has to wait after Shabbos only if a non Jew did something for a Jew on Shabbos, but if a Jew did something on Shabbos one can benefit from it straight after Shabbos because we can assume that on a future occasion a Jew will not desecrate the Shabbos for someone else. A secular Jew who does not keep Shabbos has the same status as a non Jew.
O:CH 318 Mishnah Berurah 5, Pri Megodim Eshel Avrohom O:CH 328:22, Teshuvos Minchas Yitschok 9:39, Shevet Halevi 3:54.