A turning point refers to an event, or events, in the story that bring on the actions that will eventually end the narrative.
The events that would end the narrative in A Doll's House would be:
1. The discovery of Nora's dealings, after the reading of Krogstad's first letter.
2. The reaction of the husband, Torvald, regarding such discovery.
3. Krogstad's second letter, which prompts Nora's final reaction to Torvald.
These three events are turning points that pivot the direction of the story, leading to the end.
The discovery of Nora's business with Krogstad happens when Torvald reads the letter that Krogstad leaves in the mailbox. In it, everything is exposed. What the letter may have not said is that Nora did all of those transactions out of concern and love for Torvald. That is the part that goes amiss, and Torvald does not realize it at all.
All Krogstad can say is his version of the events, which clearly paint Nora's actions as inappropriate by the standards of her time. In turn, Torvald can only appreciate what her actions mean to him: embarrassment, shame, and belittling. This leads to his reaction to the letter.
The reaction to the letter is quite telling. It shows a high degree of shallowness in Torvald, who does not once ask his wife why she did what she did. He demands an explanation from her, but, instead of letting her explain, he resorts to insults that show doubts and feelings that he may have been harboring for some time, although that is up to the reader to speculate about
Now you have destroyed all my happiness. You have ruined all my future. It is horrible to think of! [...] And I must sink to such miserable depths because of a thoughtless woman!
Torvald insults everything about Nora. He criticizes her upbringing, her father, her motherhood skills, and he also insults her, as a person. He shows no mercy, which contradicts Nora's hopes that Torvald would have put himself in harms' way for her if anything bad ever were to happen to her. This horrible disillusion leads Nora to react to Torvald, which is the final turning point that leads to the end of the play.
Nora's reaction occurs after Torvald reads a second letter from Krogstad, directed to Nora, in which Krogstad changes his mind about blackmailing the Helmers, which means that there is nothing to worry about.
Torvald's reaction is, again, quite telling. He immediately changes his attitude and, in his words, he "forgives" Nora and wants everything to remain the same. This is when Nora has enough. She explains to Torvald that she realizes now that her life has been a fake, and that she has been nothing but a plaything for everyone. She is done with everything, and she is going away, leaving her husband and children behind. In shock, Torvald tries to get her to change her mind and begs her to stay. Nora does not change her mind, which would have been a shock to society at that time, and still leaves everything without looking back.