Casually, there is another post where they suggest to do the following:

//assuming you have a ToolTip control

 void grid_MouseLeaveElement(object sender, Infragistics.Win.UIElementEventArgs e)
        {
            toolTip.SetToolTip(poolGrid, null);
        }

        void grid_MouseEnterElement(object sender, Infragistics.Win.UIElementEventArgs e)
        {
            if (e.Element is EditorButtonUIElement)
            {
                EditorButton button = e.Element.GetContext(typeof(EditorButton)) as EditorButton;

                if (button.Tag != null)
                    toolTip.SetToolTip(grid, button.Tag.ToString());
            }
        }