Did you leave the *.opf file in the OEBPS folder? It points to all the files and folders for the book, so it cannot be moved and then opened by itself, it has to remain with all the other book files.

The book might have other issues if the *.opf file cannot be opened in Kindle Previewer or doesn't work therein. Did you modify the *.opf file? You might need to ask you friend to give you the ePub file he created or whatever files he used to create the mobi file in the first place.