What's happening is, while posting a reply he creates "post reply" and the loading icon appears, but the "post reply" button doesn't grey out it is still active. While he thinks its not posted, the post is actually posted and appears for everyone. I've included a screenshot of what he is seeing, you can see its still loading the reply for him while actually it has already been posted.
Anyone have any idea how he can solve the problem? Maybe clear browser cache? Or is it really a server side/xenforo side problem?