If you want to do special hit testing to decide exactly when you want to allow sizing, you should set the insidegrid flag in the ResizingCOlsBehavior property and handle the ResizingColumns event. Here is code that only allows resizing in row 1.
private void gridDataBoundGrid1_ResizingColumns(object sender, GridResizingColumnsEventArgs e)
{
if(e.Reason == GridResizeCellsReason.HitTest)
{
int row, col;
if(this.gridDataBoundGrid1.PointToRowCol(e.Point, out row, out col)
&& row != 1)
{
e.Cancel = true;
}
}
}