Often the author will inquire how the review process
is proceeding. You can provide information on the general process,
such as how many reviews you have obtained and actions you have taken
to keep the process moving. However, you should not disclose any information
related to the quality of the paper, such as whether reviews are favorable
or your own opinion of the paper.
If the inquiry is copied to the Editor-in-Chief, then your
reply should also be copied to the Editor-in-Chief, so that he/she doesn't
also check the status of the paper.