Thank you for your research and response. While the Reset Map button does indeed allow sign in to a different portal, it also completely wipes out the existing map, so does not appear to be useful as a mechanism to recover the previously-created map.
I think a key question here is how well does ArcGIS for Excel, when a map has been created while connected using an SSO-based sign-in handle the situation when the original authorization token expires.
I'll ask the user to try, for now, to see if he can recreate his map while signed in to ArcGIS Online using a built-in account rather than signed in to ArcGIS Enterprise using an SSO based signin. That said, our organization is mandating that we shift to SSO-based signins on ArcGIS Online in the near future so a built in account might not be a viable long term solution.