We have Plot mismanagement at its height in this movie. Going into this story, the life-changing event SHOULD be Dex kissing her in the cab.
As the filmmaker, the audience is coming to the theatre because of the film's hook, Girl's low self-esteem lets her soulmate marry her best friend. Now, inherent in this premise is an eye-opening philosophical statement about growing up, throwing heavy baggage off a sinking ship, and finding LOVE for yourself.
With those three incentives, the filmmakers have to understand and master the protagonist's Character Arc and it must be STRONG! However, they chose to allow this movie to be taken over by Darcy and Ethan, the lead's best friends.