Something Borrowed

Rachel White (Ginnifer Goodwin) is a single attorney working in a New York law firm. After too many drinks on her 30th birthday, Rachel grabs a cab with her friend, Dex (Colin Egglesfield) and playfully reveals she has had a crush on him since law school. The problem is, Dex is also her best friend Darcy's (Kate Hudson) fiancÃ©. They wake up in bed together the next morning to Darcy calling both of their phones. Dex sneaks out before they can talk about what happened between them. Rachel initially believes the encounter is a one night stand, but it eventually develops into an ongoing affair. The couple soon find themselves lying to Darcy and others in order to cover up their relationship. Rachel tells Darcy that she is having an affair with another friend of hers, Ethan (John Krasinski) and an acquaintance, Marcus (Steve Howey) to cover up her sexual encounters with Dex. Ethan, who has been friends with Darcy and Rachel since childhood, is the only one who knows what has happened between Rachel and Dex, and reluctantly agrees to keep their secret.
Meanwhile, Dex's mother (Jill Eikenberry) has been suffering from depression for some time and it seems that Dex and Darcy's wedding is the only thing keeping her sane and happy. When Dex skips out on the 4th of July weekend in the Hamptons to stay in the city and spend time with Rachel, they run into his parents. His father suspects something is going on between the two of them and he tells Dex to end it as soon as possible, saying that what he wants should not be considered a priority when it conflicts with what is right. Considering his mother and father's feelings, Dex leaves Rachel in order to go back to Darcy.
As the wedding nears, Dex and Rachel are speaking less and less. At the beach, their secret is almost revealed by Ethan, who is frustrated with Rachel, especially for lying to people she cares about. That night at the bar, Rachel finally tells Dex that she wants him to call off the wedding so they can be together, but he tells her that he can't.
Ethan moves to London for a job and Rachel flies out to visit him. He reveals that he loves her and just wants to see her happy, but is sad that Rachel does not reciprocate his feelings. Rachel returns to New York to find Dex sitting on her doorstep. He reveals that he ended things with Darcy and called off the wedding.
Rachel is ecstatic until Darcy ends up at her door begging to come in to talk. Dex hides and Darcy enters her apartment. She tells Rachel that she has been cheating on him with his friend Marcus. She is also pregnant with Marcus' child and says that they are very happy. Rachel gives her her blessing and Darcy proceeds to leave but only to return and ask what Dex's jacket is doing in her apartment. Dex then comes out and Darcy figures out Rachel is the other woman. She becomes infuriated and storms out of the apartment, telling Rachel that she hates her.
Two months later, Rachel and Darcy run into each other on the street. They have not spoken since the incident and Darcy says that she is happy, but doesn't sound so convincing. Darcy sees that Rachel has picked up one of Dex's shirts from the dry cleaner, Rachel is unapologetic for sleeping with Dex but is sorry for hurting Darcy. As they leave, they turn back and glance at each other, with Darcy smiling and waving. Rachel walks around the corner to Dex sitting on a bench. They walk down the street together holding hands.
The film ends with an ambiguous scene during the credits. Darcy shows up to surprise Ethan in London; Ethan tries to ignore her and briskly walks away. The screen turns black with text indicating that the story is to be continued, which may indicate that a movie based on Something Blue, which is the book sequel to Something Borrowed,[3] will follow.

Question: What is Rachel's relationship with Dex?
Answer: He is her best friend's fiancé.