The easiest way to tooltipify a hierarchical grid is by using a separate tooltip in an item template. You can put the user control directly in the tooltip and populate it with data in the item databound event.
For your convenience I attached a sample demo which can be used as a start point for your implementation. In case you need further assistance, please prepare a sample, fully runnable demo which shows your configuration, open a new support ticket and send it to me along with detailed explanations. Once I receive it and I have a better understanding on your exact setup I will do my best to help.
the Telerik team