Well, I think this is more of a Miss Manners kind of problem. When someone in a group blurts out something so ignorant, so idiotic, so abysmally embarrassing, if the person were capable of embarrassment, the kind and polite thing to do is pretend he/she never said it at all. Turn to your neighbor and ask how his roses are doing. Has he tried Epsom salts which you've heard some gardeners use? Or, as kind Nicolau did, mention a really good restaurant meal and change the direction of the conversation. It gives the asshole, er, poor unthinking person, a chance to recover his balance and save face.