I don't get what you mean when he "replaced". Wouldn't it just be like replacing the original one which wouldn't do any harm since it's the same file, or did you mean he "deleted" the file? Which one is it, because I don't know what mediamonkey did. Never even heard or used such a program.
If he accidently delete these file, it would be very hard to recover multimedia files because they usually become corrupted after being deleted. From my experience though, I have never been able to recover any musics, pictures or videos.
Files that are deleted and overwritten are almost impossible for you to recover with softwares.