WAG: This a 800x600 screen?
Did they perhaps really use the AJAX client in 4.5.x (where you can click in any of the calendar-views to add an appointment) then are now using 5.0.x & because of the screen resolution are forced to the HTML client (where you add cal events by clicking new).
Or we talking about invites?
5.0.6+ you can use the methods listed in
Bug 28471 - option to disable redirect to lite client for 800x600 screens. to workaround the redirect. If you/hosting provider has upgraded to 5.0.9 then use the zimbraPrefAdvancedClientEnforceMinDisplay FALSE attribute on COS/account.