How does Daisy relate to this theme: "money can't buy friends"?
Daisy has money - but she has no friends. No one really knows her. Gatsby thinks he does, but in the scene in the hotel room at the end, Daisy surprises him by announcing that she does love Tom. Gatsby didn't really know her. And he didn't know that she would be turned off by his lack of reputation and status. Jordan enjoys being around Daisy, but the two don't share any intimacy. Even with her own cousin, Daisy isn't upfront and honest - she expresses some of her dissatisfaction, but quickly turns the subject, not wanting to admit that her choice to marry Tom has been so very negative.
Throughout her life, Daisy has sought money and power. As a young woman/late teen, she sought the company of those in the "country club set", flirting with men and bedazzling other girls. She enjoyed being at the center of attention, and she actively sought out those who had money. However, there is never any idea that she is close to any of the people who follow her about. She does not value personal relationships. She values status. And Fitzgerald's message is clear - that value is counter-productive and will only result in despair.
Daisy largely shows that if you have money (and good looks, and reputation), that saying isn't very true. Daisy's not a particularly good person. She's not kind, and she doesn't do much for other people. However, she was born to money and glory, and so she has all the friends of the sort she wants, her whole life. She doesn't have deep friendships—but she doesn't want them. She seeks privilege and a kind of shallow, gliding pleasure throughout her life.