Breakin' All the Rules (2004) on Netflix

Plot summary

For Quincy Watson, heartbreak becomes a career: after his fiancée Helen Sharp ends their engagement at the party celebrating it, the letter he writes to her turns into a how-to book on ending relationships. It sells, and Quincy starts handing out breakup advice, including to his boss Philip Gascon, who wants out of his relationship with Rita Monroe. Quincy's cousin Evan Fields reads the book and begins to doubt his romance with his girlfriend, Nicky Callas. Convinced Nicky is about to leave him, Evan asks Quincy to talk her round, describing her long black hair without knowing she has just had it cut short. At the bar where Nicky was to meet Evan, the two strangers fall into conversation, and she gives him a false name.