DEAR MISS MANNERS: We have friends, a married couple, with whom we have vacationed on several occasions. We live in different states, so we don’t see them that often. This past Thanksgiving, we rented a house with them for a week. It was the week from hell. They fought constantly, trading insults; at one point, the wife was so upset that she left the restaurant right after we had all ordered food. It was a long, tense dinner at an expensive restaurant. Her husband can be very insensitive to other people. It’s usually all about him. My dilemma is that I don’t know how to tell my friend that it …