If you are not running Rollup 12 then you need to set their machine to IE10 Compatibility mode.
Rollup 12 resolves the issue as it was released after IE10
Here is the KBA on the subject
support.microsoft.com/.../2743941
Currently CRM Online is on UR10, so it will only work with the desktop IE10 on Windows 8 and only in Compatibility Mode, so the CRM website must be added to the list of sites to run in Compatibility Mode.
With the December 2012 Service Update the requirement for Compatibility Mode will be removed but it will still only support the desktop IE10 not the modern UI version of IE10.