I agree with all the cautionary comments. It's very likely that there would not be a problem reapplying, but it certainly not worth the risk. Depending on her nationality it may make a difference, since US visa policies vary dramatically by country. Leave early, 2-3 days early, or get an extension. IN an identical situation with my niece a couple fo years ago we got a two week extension in about a week. Visit your nearest INS office to do this. There are long lines at most of them.