Have you got a URL where I can test this myself?
Failing that, if they press F12 in their browser (preferably a recent one such as Chrome or Firefox) there will be a "Console" tab. Clicking on that should detail any script or resource errors.
It would be great if they could copy and paste the content of that. It might tell us what is going on.
Also, what browser are they using? Is it common to a specific browser and version?