It is forbidden for two acquaintances to eat at the same table if one eats meat and the other milk or dairy products. The Sages were concerned that the acquaintances might share each other’s food and thus violate the prohibition against partaking of meat with milk, and they therefore forbade eating meat at the same table where somebody partakes of milk, and vice versa. (See Shulhan Aruch, Yore De’a 88). Acquaintances may eat meat and milk at the same table only if they place on the table a "Heker," or some object that is not normally on the table (such as a vase), to remind them that they may not partake of each other’s food. If the table is long and the two acquaintances eat far enough from each other so that one cannot reach the other’s plate, then they may eat together at the same table, even without a "Heker."
The question arises as to whether these Halachot apply to a person who eats meat alone at a table containing dairy foods (or vice versa). In this case, there is no other person with whom he might share food, but on the other hand, we should perhaps still be concerned that he may mistakenly partake of the dairy foods.
Indeed, Halacha forbids eating meat alone at a table containing dairy foods (or vice versa), even if one places a "Heker." In the case of two acquaintances, placing a "Heker" allows them to eat together because they have two safeguards: the "Heker," and each other. Meaning, even if one of them overlooks the "Heker" and wishes to partake of the other’s food, the other person will likely stop him. In the case of a person eating alone, however, the "Heker" serves as his only safeguard, and this does not suffice to permit eating with dairy food on the table.
Hacham Ovadia Yosef added, however, that one may eat meat at a table containing dairy foods if another person is present. Even if that person does not eat anything, his presence suffices as the second "safeguard" which allows eating meat at a table containing dairy foods.
The Peri Megadim (work by Rabbi Yosef Teomim, 1727-1792) noted that this prohibition does not apply to a person who had eaten meat within the previous six hours. Even though such a person may not partake of dairy foods, he may eat Parve food at a table together with somebody eating dairy foods. The Peri Megadim maintains that the Sages did not go so far as to forbid eating at a table with dairy foods within six hours of eating meat. Likewise, it would be permissible for somebody to prepare a dairy dish within six hours of eating meat, provided, of course, that he ensures not to taste any of the dairy food.
Summary: One may not eat meat at the same table where an acquaintance eats dairy foods (or vice versa), unless they are seated beyond arm’s length from each other, or they place on the table an item that is not usually on the table. A person eating alone may not eat meat if dairy products are on the table (and vice versa), even if he places an object on the table. Within six hours of eating meat, one may not eat dairy foods, but he may eat Parve food at a table with an acquaintance eating dairy foods. One may prepare dairy dishes within six hours of eating meat.
