The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Shemini (22), discusses the laws relevant to a husband and wife that eat together when the wife is a Nidda (listen to audio recording for precise citation). He writes that a husband and wife may not eat together at a table when the wife is a Nidda unless they make some Heker (literally "indication"). This is done by having some unusual feature at the table, such as placing an extraneous object in between them on the table. For example, they can place a loaf of bread that they are not using for the meal, or a pitcher that they are not using, on the table in between their plates. According to some views, it suffices for one of them to sit in a different place at the table than usual. Some authorities also allow them to eat together at their regular places if they use separate placemats, or if one eats directly on the table and the other on a tablecloth. According to some authorities, this Halacha only applies when the couple eats alone. If they eat with their parents, for example, they do not need to do anything unusual as a reminder that the wife is a Nidda.
According to all opinions, the Heker must be something unusual, which is not normally done. Therefore, although a Heker is not needed when the couple eats together with other people, they do need a Heker if they eat with their children, since this is a usual manner of eating. Similarly, if a man customarily places his cell phone on the table when he sits down to eat, then the phone does not suffice as a Heker, since it is normally on the table at mealtime. By the same token, a vase or saltshaker that always sits on the table cannot be considered a valid Heker. It is acceptable, however, to place ones wallet on the table, or a walky-talky, or similar objects that are not usually placed on the table. The item used for the Heker should be large enough that it is readily visible. A ring, for example, should not be used as a Heker. According to some authorities, the object must be at least one Tefah (approx. 3 inches) high.
Summary: When a wife is a Nidda, she and her husband may eat together only if they make something unusual at the table, such as by not eating in their usual seats, using separate placemats, or placing in between them an object that is not normally on the table, such as an empty pitcher. This is not required when they eat together with other people, but it is required when they eat with only their children.
