The Mallory Effect (2002)

"Get dumped. Get replaced. Get mean."

Charlie can't take his mind off of his ex-girlfriend, Mallory. When he sees her with her new boyfriend, his obsession takes charge. With the cut-to-the-chase advice of his best friend Nick, Charlie decides to take action to recover his lost love. His plan: secretly befriend her new boyfriend and sabotage their relationship from the inside out.