The CellFormatting event is the right place for this - you are on the right path. The only thing you need to set is the BorderBoxStyle property as well:
e.CellElement.BorderBoxStyle = BorderBoxStyle.FourBorders
e.CellElement.BorderBottomColor = Color.Red
e.CellElement.BorderBottomWidth = 1
e.CellElement.BorderTopWidth = 0
e.CellElement.BorderLeftWidth = 0
e.CellElement.BorderRightWidth = 0
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