In a case where there is no available prepared food on Shabbat for an ill patient who needs to eat, or if he requires a particular kind of food which is not available, it is permissible to ask a non-Jew to prepare the required food for the patient. Even though the patient’s life is not in danger, nevertheless, the Sages waived the Rabbinic prohibition of "Amira Le’akum" – asking a gentile to perform forbidden activity on Shabbat – for the benefit of an ill patient, order to ensure that his condition does not deteriorate. And, although the prohibition of "Bishul Akum" forbids eating food cooked by a non-Jew, the Sages waived this prohibition, too, in the case of an ill patient on Shabbat, when Jews are not allowed to cook.
The Shulhan Aruch (Orah Haim 328:12) rules that in such a case, where a non-Jew was asked to prepare food for an ill patient on Shabbat, this food becomes forbidden once Shabbat ends. Meaning, if the patient does not eat all the food, the remaining food becomes forbidden on Mosa’eh Shabbat, since at this point it is permissible for Jews to prepare food, and there is thus no reason to waive the prohibition of "Bishul Akum." Although some Halachic authorities permit eating the leftover food on Mosa’eh Shabbat, Halacha does not follow this opinion. However, the utensils used for preparing the food remain permissible, despite the fact that a gentile cooked with them. Normally, if a non-Jew cooks food in a pot, the pot becomes forbidden by virtue of the fact that it absorbed food which is forbidden due to the prohibition of "Bishul Akum." In this case, however, since there are some authorities who, as mentioned, permit even the leftover food after Shabbat, we will not go so far as to forbid the use of the pot after Shabbat.
The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) notes in this context that although the Sages waived the prohibition of "Bishul Akum" for the sake of an ill patient on Shabbat, this does not mean that other Rabbinic prohibitions are similarly waived. For example, an ill patient who feels he needs chicken with cheese may not eat such food, despite the fact that it is forbidden only by force of Rabbinic enactment. (The Torah prohibition against eating milk with meat does not apply to poultry; it was the Sages who enacted the prohibition against eating poultry with milk.) The Sages permitted "Bishul Akum" for an ill patient because the food is intrinsically permissible, and it is only due to its having been prepared by a non-Jew that the Sages instructed us not to eat it. Intrinsically non-kosher food, however, remains forbidden for ill patients, even if its prohibited status was enacted by the Sages.
Summary: If an ill patient needs food prepared on Shabbat, a non-Jew may be asked to prepare food for him, but the food becomes forbidden after Shabbat. The utensils in which the food was prepared remain permissible even after Shabbat.
