Whom does Maximilian overhear talking in the garden in chapter 41?

After learning that Valentine is betrothed to M. d'Epinay, Maximillian bids his love au revoir and returns home.  The next day he receives a note from Valentine, telling him that her heart is his. He secures a ladder and returns to the gate of the garden to await his love.  Instead he overhears the Doctor d'Avrigny speaking to M. de Villefort.  He informs the prosecutor that his mother-in-law did not die naturally; she was poisoned.  The doctor refers to the death as a "mistake" and bids M. de Villefort to "make inquiries."

After hearing this, Morrel goes to the house whose door happens to open [the Count at work] and he finds Valentine upstairs in her grandmother's room.  Valentine takes Morrel to meet her grandfather, Noitier, who instructs Maxmillian to not pursue the elopement plans; Noitier promises that he will foil the marraige to Franz d'Epinay.  Maxmillian swears that he will trust Noitier and leaves the house.