GUEST,US, visas expire sometimes for any number of reasons, and the person who holds the visa doesn't always have any power to prevent that from happening. It's the responsibility of the visa holder to leave the country before the visa expires (if they have not been able to get it renewed), but it's not always an easy thing to do. US immigration law is a very complicated thing, and there's just no way you're going to understand it here in this thread, so you might as well not even try.