If somebody at a dinner party hardly touches their food, should I ask them if they didn't like it? That's what they do in restaurants, but my girlfriend says it's rude.
As with any situation involving friends around a table, it's a matter of context. Is this your former glow-in-the-dark-naked-Twister-league partner we're talking about or your new boss's uptight second husband? Is the setting light and casual or more formal than that? Either way, I'd probably ask: quietly and discreetly in a more formal situation, loudly and accusingly at a casual dinner with friends. ("What, so now you're too good for my tempeh and mung sprout soup, punk?") Whatever the problem, your job as host is to try and show your guests a good time. Maybe that means fetching them an aspirin, offering something else to eat (within reason, of course) or getting them a fresh plate without the half-eaten tomato hornworm on it. But you're never going to know until you ask.