153:6 When a woman is a niddah, she and her husband may not eat at the same table without some sort of visual reminder, such as placing something in between their plates that is not normally on the table at other times. Alternately, she may change her seat. If they normally eat from the same dish when she is clean and they eat from separate dishes when she’s a niddah, that is a sufficient reminder.
153:7 The husband may not drink from what remains in her cup, though if another person drank in between them, or if the contents were poured into another cup, then it is permitted. If he doesn’t know that she drank from that cup, one need not inform him. However, if he is aware that she drank from it, but he doesn’t know that she is a niddah, then one should inform him. The wife is permitted to drink what remains in the husband’s cup. Some authorities say that it is also prohibited for the husband to finish his wife’s leftover food.