You can inherit sale.order add field "confirmed_by",

confirmed_by = fields.Many2one("res.users","Confirmed By")

 Set confirm by users on confirmed button

  @api.multi

    def action_confirm(self):

        for order in self:

   --- add this line ---

   order.confirmed_by = self._uid  # set user who confirmed sale order

    ---then rest method ---

  add confirmed_by field in to Tree view

You can see what happened to a document in the chatter history below of every document. To see it in the tree view you would have to create a module extending the existing tree view, but be aware that as time goes by a document can be confirmed multiple times by multiple persons.