You need to show the tooltip that corresponds to the appropriate row, not call the manager's show command. Doing so will automatically trigger the AjaxUpdate event. How to do that is exaplained in the following demo on the ToolTIpManager's client-side API: http://demos.telerik.com/aspnet-ajax/tooltip/examples/radtooltipmanagerclientapi/defaultcs.aspx. More information on how to pass the needed value to the user control is available in the following remo: http://demos.telerik.com/aspnet-ajax/tooltip/examples/targetcontrolsandajax/defaultcs.aspx.
For your convenience I created and attached a simple page demonstrating this functionality. Here is a video with the expected behavior: http://screencast.com/t/Xbup6VYNpkZ.
I hope my reply was helpful.
All the best,
the Telerik team