Doubly unusable if he moved the document
If he started the document under LibreOffice (which I suspect he did) it was doubly unusable. It is somewhat specific to Office for Mac X (dunno what MSFT did when they ported it). I have had to throw out and rewrite from scratch multiple documents because of this. The scenario is: a document that once in its lifecycle has passed through LibreOffice, gets to a genius with Office for Mac who does a tracked change on it. Merry hell ensues with 100% or so probability. The same scenario just with lower (5-10%) probability can also play with Office for Mac and Offie for Windows. The document from now on is a mess on anythying - Office for Windows, Office for Mac or LibreOffice.
Still, the platform-to-platform breakage in tracking is nothing compared to breakage in references, citations and indexed tables.