Something Borrowed (2011)

Rachel White, a thirty-year-old single woman who is a consummate good-girl. She and Darcy Rhone have been best friends since childhood, and hard-working Rachel is often in the shadow of flashy, sometimes selfish Darcy. Then, after a night of drinking on Rachel's thirtieth birthday, she sleeps with Darcy's fiance, Dex. After this turns into an affair, Rachel explores the meaning of friendship, true love and ethics.