Ah, sorry, misread your post and though you were after a double-underline.

Have some VB by way of an apology

Private Sub Worksheet_Change(ByVal Target As Range)

Const sTRIGGER_CELL As String = "C5"
Const sSTART_CELL As String = "C15"

If Not Intersect(Target, Range(sTRIGGER_CELL)) Is Nothing Then
  Range(Range(sSTART_CELL), Cells(Range(sSTART_CELL).Row, Columns.Count)).Borders(xlEdgeBottom).LineStyle = xlNone
  Range(Range(sSTART_CELL), Range(sSTART_CELL).Offset(0, Range(sTRIGGER_CELL).Value - 1)).Borders(xlEdgeBottom).LineStyle = xlDouble
End If

End Sub