Your customer's users are probably trying to concurrently open the same file with read/write access. Windows does not allow this because it can cause the users to have inconsistent (and possibly corrupted) views of the file. If the users are not changing the files, it may, depending on the application, be possible to work around the problem by marking the share permission as read-only. This Microsoft support article has the necessary details.
This was first published in June 2007