David,
I've seen this error before when there is something corrupted in the document, or (I think) with different account permissions. I believe it occurs when Docs can't access or have permission to access an element of the document.
Here are two things to try:
1. Make a copy of the document (using Drive's "Make a copy" menu item). I believe the copy action does more than simply duplicate the file, it makes new entries in the underlying object store for all element blob resources (like images). In other words it does some integrity checking when it does the copy.
2. Have your client use the new copied document. If they see the error again, ask them to check what Google account is signed in. The issue may be related to security permissions on the account or if they've accidentally used a personal account to access the doc.
Hope this helps, there is not a lot of documentation that helps debugging issues like these. The info above is my guesswork from trying to figure out similar issues