I think we can safely conclude that the other characters of this excellent play, in particular Krogstad and Mrs. Linde, are most definitely responsible for Nora's downfall. It is clear in Act Three of this realist drama that things are catching up with Nora. Krogstad is threatening to reveal the truth about her spending habits fo Helmer, and there appears to be no escape. However, Mrs. Linde does have the ability to stop Krogstad from sending the letter, but she believes that Helmer should know the truth about Nora and her actions. Note what she says to Krogstad:
Helmer must know the truth. This unhappy secret of Nora's must be revealed. They must come to a full understanding. There must be an end of all these shiftings and evasions.
Mrs. Linde clearly feels that the artificiality and deceit of the relationship of Helmer and Nora must come to an end, and she sees Krogstad's letter as an excellent way of bringing things to a head and forcing Nora to grow up and mature. Clearly, therefore, Nora's downfall, if you choose to call it that, can be said to be the responsibility of other characters who are choosing to act in a way that they thing will help Nora.