The reason so many people adhere so strictly to gender norms is not because they’re inherent, but because they are policed in exactly the way that woman’s daughter is experiencing. Gender nonconformity is unsettling to people because we have internalized the “rules” of gender. It takes conscious thought to overcome being unsettled. I’m guessing this woman was highly gender conforming as a youth and the experience of thinking about gender or having it questioned is (or was) very strange to her when the topic was injected into her life by her daughter. Now she sounds surprised that it’s new to other people!
How will she cope if her daughter comes out as trans sometime down the road!