I don't have an answer for your user's problem. I do have a comment, though.

Your user should not be opening documents attached to email directly from email. That is one of the most frequent causes of lost work because such files are saved in temp folders when the user presses Ctrl+S. Instead, they should save the document to a local location (or OneDrive) and open the already saved document from there.

Sometimes all they need to do is look at a document, they don't actually need to save a copy, or make any changes. But otherwise, I do tell them it's better to save the attachment rather than open and save as. But thanks for the reply, in any case. This one has me stumped.