They only think they are right.
It's your job as the site designer to inform them of the pitfalls and common misconceptions (one of which is they can edit the site themselves to add new content at will)
[1] Tell your client that auto-playing advertising videos are turning people away
[2] The fact that I have never been to your URL before and that all links worked in all browsers means that the site is functioning fine.
If they don't work from another computer, then there is a local issue. So you need to look at the local PC and find out what it's problem is there (addons, malware, missing updates, hardware failures.... etc)
You could also tell him to update to use a proper web browser
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