Anything in your server error log?
Would need to know their browser version, OS version etc.
Also, do they have any browser add-ons installed? Such as ad-block or anything else that may control javascripts such as other browser add-ons or security software on their PC?
If they registered an account here, can they post in the test messages forum? Or is it only your forum?
Can you provide a screenshot of their browser console output? There may be other indications in there as to what's going on.