I'm not making a value judgment on Nora's cheating or assigning blame. I'm talking about foreshadowing and analyzing the events as a storytelling convention. If I were to attempt to diagram problems with the relationship between Alex and Nora (who I must admit is probably my favorite love interest, issues notwithstanding) I would assert that women who cheat on their partner will tend to cheat on any future partner as well (men too, honestly). At any rate, I wasn't discussing that so if you wish to refute it feel free. The foreshadowing aspect is only a possible allusion to future events. No other love interest is as intertwined with prospects of marriage, children, infidelity, and divorce as Nora is. They stand up to get married. They raise a plastic baby together. Nora cheats, either physically or emotionally is your choice. Nora's parents are in the throes of a divorce. It isn't exactly subtle.
Also, if you didn't pick up on the fact that Chloe and Nora hate each other you are NOT paying attention. She might not have said "I hate you" but boy do actions speak louder than words.