You should use MouseDoubleClick instead (better because it gives you the point that was clicked, and it is expressed in client coordinates). In the event handler, you could cast the sender to an UltraGrid, get the UltraGrid.DisplayLayout.UIElement, and use ElementFromPoint to find the RowUIElement that was double-clicked (if any).

Example:
void ultraGrid_MouseDoubleClick(object sender, MouseEventArgs e)
{
    UltraGrid control = sender as UltraGrid;
    UIElement controlElement = control != null ? control.DisplayLayout.UIElement : null;
    UIElement elementAtPoint = controlElement != null ? controlElement.ElementFromPoint(e.Location) : null;

    UltraGridRow row = null;
    while ( elementAtPoint != null )
    {
        RowUIElement rowElement = elementAtPoint as RowUIElement;
        if ( rowElement != null )
        {
            row = rowElement.Row;
            break;
        }

        elementAtPoint = elementAtPoint.Parent;
    }

    if ( row != null )
    {
        //  If you get in here a row was double-clicked.
    }
}