Thank you for the additional information. I would first clear the browser cache on all browsers, just to rule that out.
Then, make sure they did not perform a Host File Modification, which will affect their connectivity to the site.
If you have access to the server hosting the website, then I would tail the access logs, while they are requesting the .css file. This should provide a more specific answer. If you do not see the user's request, they may not be resolving to the proper server, possibly due to propagation.
If you have any further questions, feel free to post them below.