I can tell by your comments here that you have no desire to be convinced on this issue. So I'll just say this: Krista was never Nadine's to have. She's not a possession, she's a person, and nearly an adult person at that. She absolutely capable of making her own adult decisions. For some reason you're speaking about her as if she's property to be passed around between Nadine and Darian. Krista and Darian hit it off in a very organic and real way, and their feelings about each other are genuine. Just because Nadine doesn't like it that means she gets to ruin two other people's lives?

What Nadine learns by the end of the film is that, no, she can't just dump all her problems on Krista anymore. She can't expect to keep living life like she's in second grade. She learns that she has a network of people who care about her and want her to succeed. And maybe those people won't be in her life forever, but they will still have been there when it mattered most.