You can have different rights as a user connected in your work's network vs someone on the outside. You might have access to a CSS file/ JS file that was mis-coded, but since it sits on your network you can get to it.
Other things are you can have your browser set up to trust your website. [You can have it in a different internet zone.]
Maybe
you should ask the user that has the issues to grab the view source and send it to you. That might tell you a few things.
Eric