In general, it is prohibited to benefit from a violation of Shabbat, even if done by someone else. The Poskim discuss a case in which a guest at the Shabbat afternoon meal noticed that his host instructed the non-Jewish housekeeper to turn on the gas stove and heat up food taken from the refrigerator, a clear violation of Amira L’Akum (Telling a Non-Jew to perform Melacha on Shabbat). The question is whether it is permitted for the guest to eat that food, which was heated in a prohibited manner.
The Poskim rule that he may eat it only after it has cooled down to room temperature. At that point, any added benefit from the violation is neutralized, since the food had already been cooked before Shabbat and the only Melacha was heating it. This would also be true in a case in which she reheated cold liquids, such as a soup. Even though heating cold liquids is considered re-cooking, and not just reheating, the guest may partake of the food after it has cooled down; it has then reverted to its prior status.
Importantly, this applies only if the food cannot be eaten cold. If one instructed a non-Jewish housekeeper to reheat such a food on Shabbat, then it may not be eaten until it cools. But if the food can be eaten cold, then even if it is more enjoyable when eaten warm, it may be eaten immediately after it was heated by the housekeeper. (See Halacha Berura, vol. 14, page 140.)
However, the status of the host who actually instructed the non-Jew is stricter. The Halacha penalizes the host for instructing the non-Jew and may not partake of the food, even cold, until after Shabbat.
While having household help may be a Beracha of convenience, one must be careful not to fall into the trap of using non-Jews in violation of the Shabbat. The basic rule, although somewhat oversimplified, is that one may instruct the non-Jewish housekeeper to do only those tasks that are permitted for a Jew to do on Shabbat.
Food reheated by a non-Jew in violation of Shabbat, may be consumed by a guest after it has subsequently cooled down, but may not be eaten by the host who instructed the non-Jew until after Shabbat.
One must be very careful to use non-Jewish housekeepers only for tasks that are permitted for a Jew to do on Shabbat.