Is it permissible, when we offer condolences to people, to have dinner at their place?

There is no harm in preparing food for the family of the deceased and comforting them with it, as this is commendable. However, if the family of the deceased prepares food for people, this is an innovated Bid'ah, and its ruling oscillates between prohibition and severe reprehensibility, and it has been considered a form of Niyaha (wailing).