Have you examined the following help article that treats this problem: http://www.telerik.com/help/aspnet-ajax/window-troubleshooting-missing-borders-titlebar-and-commands.html
? Could you give the suggestions there a try and see if they help? Also, if the behavior is isolated to only one user the issue may be localized there - browser settings, proxy server, internet connectivity, a firewall, etc. may be blocking some of the webresourecs (namely the image sprites).
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed