When a married woman becomes a Nidda, Halacha imposes several precautionary measures to safeguard against sin. These measures, which are intended to create a certain distance between husband and wife during the Nidda period, are referred to in Halachic jargon as "Harchakot" (or "distancing measures").
One of the Harchakot relates to a married couple that eats alone at the same table during the period of Nidda. Halacha requires that the couple make some discernible reminder while eating that the wife is a Nidda, and this can be achieved in several different ways. First, they can place on the table an object such as a vase which is not normally kept on the table. The object should be visible by both of them during the meal. Alternatively, if a tablecloth is spread on the table, either the husband or wife can lift the tablecloth on that end of the table and eat directly on the table. This, too, suffices to serve as a reminder of the wife's status. Likewise, if a husband and wife normally use placemats while eating, one of them can eat directly on the table without a placemat. Additionally, if each has a place where they generally sit while eating, they can sit in different seats. If the husband and wife sit at a distance from one another, such that they cannot reach each other, then no reminder is necessary, and they may eat together alone. (See Taharat HaBayit)
These Halachot apply only when husband and wife eat alone at a table. If family members or guests are present at the table, then they do not require any reminder. At a restaurant, however, where the husband and wife sit alone at a table, these laws do apply, despite the presence of other guests in the restaurant. They must therefore ensure to place an object such as a cellular phone on the table to serve as a reminder of the wife's Nidda status.
Summary: When a wife is a Nidda, she and her husband may not eat alone at a table unless they make some deviation from their normal routine, such as by placing an object on the table, having one eat on a tablecloth or placemat and the other directly on the table, switching seats, or sitting at a distance from one another. If other people sit together with them at the table, this requirement does not apply.
