The narrator seems so terrified by the sight of Madeline Usher that it seems highly unlikely that he was aware that they had buried her alive. When he first hears the sounds of Madeline approaching, the text mentions that the narrator is "completely unnerved" and "leaped to [his] feet." The sounds of her movement have taken him fully by surprise. He rushes to where Roderick sits. When the doors open, the narrator notes:
...then without those doors there did stand the lofty and enshrouded figure of the lady Madeline of Usher.
That Poe placed the emphasis on "did" by putting it in italics stresses that the narrator is trying to come to terms with the sight of Madeline himself. The narrator finds the sight of Madeline, alive and walking in through the doors while wearing a shroud, incredulous.
Madeline falls on Roderick, which causes the narrator to react in terror: "from that mansion, I fled aghast." He doesn't make any attempt to save Roderick because he did not see this coming. As Madeline approaches, he is quite frozen to the spot in terror and can only think of saving himself as the House of Usher literally falls around him.
Evidence indicates that Madeline's reappearance, covered in blood and showing evidence of a "struggle" in her escape efforts, catches the narrator completely off guard. His shock and horror at finding her alive indicates that he did not realize that she was still living when he and Roderick laid her to rest in her tomb.