In the old days of banquetting, it was considered rude to raise your elbows above the level of the table while eating (unless you were taking a drink) because doing so might knock someone elses' food off the table, or you might get your elbow in their food, or you might elbow them in the face while you were cutting your meat or something.... . On tables with a little more space between people, there's not much wrong with putting your elbows on the table, so long as it isn't some really fancy dinner thing.. . So, yes, it really is rude to put your elbows on the table... sometimes... but not nearly as rude as it is to reach across the table to push someone's elbows off the table.. . Reaching over a table and pushing elbows off is considered a public disgrace of both members, and the only way to properly handle such a serious affront would be to duel with pistols at 30 paces until one party or the other is dead... but hopefully your friends can come up with a less murderous way of apologizing to each other.