Hi Holger,
Thank you for contacting Syncfusion Support
Currently we don't have Click event for Help button which get created while applying Office2013 theme. But we have created a work around sample for your requirement. In the sample we have fetched HelpButton using VisualUtils and hooked the click event for this Button.
Please download the sample from the below link
Sample link:
RibbonSamplePlease let us know if you need any other assistance
Regards,
M.Sugapriya