I would try getting him to browse to this url: https://djmgardenfurniture.co.uk/wp-content/themes/Untitled/style.css
Then when that has loaded, try viewing the site again. If that doesn’t work, clear his cache and try again.
It could also be a sign of spyware on his computer as it could be redirecting the CSS URLs to something else and so none of the CSS is successfully being loaded.
