Harper: "I want your credit card. That's all. You can keep track of me from where the charges come from. If you want to keep track of me. I don't care" (5.8.2).
Joe tries to come back to her, but Harper isn't having it. In the end, it's Harper who abandons her husband, taking his credit card and heading off to San Francisco. Joe's abandonment, in the end, seems like the best thing that could have possibly happened to Harper. She's now found the strength to live without him. We only hope Joe finds the same thing.
Prior: "I love you Louis. [...] I really do. But you can't come back. Not ever" (5.8.15-17).
Prior refuses to take Louis back; he can't forgive his betrayal. In the Epilogue, we see that Prior and Louis are still friends, even if they're not lovers, showing that their relationship has evolved into something new and hopefully better. It could be that Louis' abandonment of Prior ended up being good for both of them... maybe. [Do we know for sure that they're just friends at the end?]