Yes, this can happen. Its never happened to me personally, to my knowledge, but a virus can hide itself in a file in any number of ways, and can be programmed to be invisible until a certain date, or until certain actions are performed. Your friend’s concern is valid. If there were a virus, worm, or any other malware, it would hide inside a Word file, an email file, a picture file, a video file, or an audio file. I’m sure there’s other ways too. However, if your USB drive is empty, (be sure the option to show hidden files is checked so you really know whether its empty or not), then your friend really should not be concerned. An empty USB (empty of hidden files and all files) is harmless, far as I know.