Swimfan

Ben Cronin is a star swimmer of his high school's swim team. His coach informs him that Stanford University scouts will appear at next week's swim meet. Ben and his girlfriend Amy discuss their future plans. Amy desires to attend school in Rhode Island but explains she will go to school in California to stay close to Ben. The next day, Ben nearly runs his car into Madison Bell and gives her a ride home as an apology. Later, he realizes that Madison left her notebook in his car. The notebook is filled with music notes, and Ben spots his initials written inside a staff. When he returns the notebook, he meets Madison's cousin, Christopher. Madison appears stressed and explains she has not eaten, so Ben offers to take her to dinner. At dinner, Ben tells Madison about his girlfriend, but Madison does not appear too bothered and explains that she has a boy waiting for her in New York City.
Ben shares some of his past with Madison; he began doing drugs five years ago, which led to crime and six months in juvenile hall, which "saved him" because he ultimately realized his passion and talent for swimming. Although Ben tries to end the date, Madison convinces him to go to the pool. Her aggressive flirtation lures Ben in, and despite his initial hesitancy, the two have sex. Both agree to remain friends and not to discuss their encounter.
The next night, Ben goes to a party at Amy's house. Amy introduces Ben to her new friend, who turns out to be Madison. The two pretend to have not met one another. Shortly after, Madison obsesses over Benâshe stops by his house to meet his mom and bombards Ben with e-mails and instant messages. Ben realizes her unhealthy behavior and demands she leave him alone. Ben's lying eats at him, but, before he confesses, Madison tells Amy first. Madison dates Ben's rival teammate, Josh. Right before their biggest swim competition, Ben is disqualified for having steroids in his urine. Ben is outraged and suspects Madison had Josh set him up. Ben confronts Josh about the drug test, revealing his suspicion. Days later, Madison accidentally calls Josh by Ben's name while they are kissing in a car. Josh realizes that Madison's obsession with Ben is real and tells her off.
Ben tries to tell Amy everything, but she disbelieves him. The next day, Ben goes to the pool, where he finds Josh dead. The police suspect that Ben is to blame for Josh's murder. To prove his innocence, Ben breaks into Madison's room to find evidence. He discovers a bottle of steroids and a creepy shrine of his personal belongings she has been secretly stashing. Christopher warns Ben of a similar case regarding a man named Jake Donnelly. When Ben visits Jake in the hospital, a nurse tells him that Jake's girlfriend Madison survived the crash.
Madison disguises herself as Ben and steals his car. She follows Amy home from school and runs Amy off the road, and again Ben is framed for the crime. That night at the hospital, Ben and a few friends record as Madison confesses her crime and intentions, resulting in Madison's arrest. She escapes custody by stealing an officer's gun and shooting the two policemen escorting her. She enters Ben's house and forcibly takes Amy to the school's swimming pool. As Ben watches, Madison throws a handcuffed and chairbound Amy into the pool. Madison attacks them with the handle of a pool cleaner, and Ben grabs one end, pulling her into the pool. As Madison does not know how to swim, she drowns while Ben frees Amy from her handcuffs and carries her out of the pool. Ben resuscitates Amy via mouth-to-mouth. Later, at a swim meet, Ben is a spectator. He goes outside to his car, where he and Amy kiss and drive away.

Question: Who disguises herself as Ben?
Answer: Madison