My family was recently invited to friends for Shabbat lunch. We noticed that they had reheated food in an oven (not clear if it was on a time or had been left on all of Shabbat). Is it permissible to eat food that was heated up in such a way? What should one do in such a situation? Also, some of the food had a lot of liquid (i.e. meat with a lot of gravy). Is that also problematic to reheat on Shabbat?

Shalom, There do exist certain ovens that are made in such a way as to allow their use for reheating on Shabbat. But in general it is forbidden to reheat food in an oven on Shabbat. However, the laws of benefiting from forbidden activities on Shabbat are more lenient than the laws of how to act in the first instance - therefore, fully cooked food that was heated up in an oven, even if it was liquid, may be enjoyed on Shabbat. This is true even if you are unsure if they sinned by turning the oven on. If the oven was definitely turned on in a forbidden way on Shabbat one should refrain from eating the food whilst still hot - unless for social (or other) pressing reasons one needs to rely on the most lenient of opinions and eat it. The best way to act depends on your relationship to your hosts - if you feel you are in a position to "educate" them nicely, and they will accept it, do so after Shabbat. Otherwise, it is a mitzvah to remain silent in the face of those who will certainly not listen.