Q. 1) A baal teshuva who lives with his not-yet-frum parents – the parents sometimes go shopping on Shabbos and buy food, including food that they buy specifically for the baal teshuva. How soon after Shabbos, if at all, may the baal teshuva eat this food?
(One who desecrated Shabbos on purpose may not benefit ever. Here, the baal teshuva is not desecrating Shabbos, but his parents are desecrating it specifically for his sake.)
2) A married couple where the husband became a baal teshuva, what should he do with food on Shabbos, until his wife joins him?

A. Shulchan Aruch (O.H. 318: 1) rules that if prohibited work was intentionally done on Shabbos for someone, the recipient may not derive any benefit from that forbidden activity until the end of Shabbos.
Biur Halacha (ibid.) maintains that the above applies even on rabbinical prohibitions. (Although he also quotes the disagreeing opinions of Hagra and Chaye Adam). Shulchan Aruch asserts that it is permitted immediately at the end of Shabbos and one does not have to wait until the time it would take to perform anew that melacha after the end of Shabbos; a condition usually needed when a Gentile performs a melacha for a Jewish recipient. However, Horav Shlomo Miller’s Shlit’a opinion is that in this case it may apply to everyone, since the reason for the difference, cited by Mishna Berura (ibid. 5) is that having a Gentile do work for one’s sake is seen as a weak prohibition and it may become a common occurrence. In this particular case of the parents transgressing for the benefit of their son may also become a constancy.
On question 526 in regards to a recent baal teshuva who’s parents are still non-orthodox but very suportive of their son and insist on he spending Pesach, as always together. I quote from the question; I came to an understanding with my parents and they are letting me use a small kitchen in the basement that I kashered. I buy my own kosher food and use disposable dishes. But for the Seder, I have to join the family and the food is not kasher lepesach or otherwise. Can I sit with them, place a napkin and eat my own food and recite together their shortened version of the hagadah?
A. Shulcha Aruch (O.H. 440: 3) rules that you are not to seat on Pesach at the same table with one that is eating chometz even if you are using a separate tablecloth. Mishnah Berurah (ibid. 18) explains that although in other prohibitions this may be permitted, chometz in Pesach is different since even a very small amount is forbidden. The best scenario ideally would be if you could convince your parents to observe a proper kosher seder. Since that may not be possible yet and I presume you are trying to change their attitude to religion by being kind, cooperative and respectful your best bet may be to go along with the following guidance from Horav Shlomo Miller Shlit”a. Since I presume you are in the Toronto area the shkiah this year at the first seder will be at 8.00 PM. Most likely your parents will make an haalachik unacceptable early seder before Pesach even begins. At that time you can still eat without making kidush. You can eat your own kosher l”pesach food sparingly so you don’t satiate, but not matza. You can also recite and sing along with your family their shortened Hagadah English version, explain to them the real meaning of Pesach and finish before shkiah. Then when night falls you can either do your own private seder or better get invited to a proper seder. Similarly, in your case although he cannot partake from the food his parents or his spouse prepared illicitly on Shabbos, for the sake of Shalom Bais and eventual kiruv of his parents or wife, he should sit together with them, sing zemiros and say divrei Torah, while he consumes his own food.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a