Donnie Darko

On October 2, 1988, Donnie Darko, a troubled teenager living in suburban Virginia, is awakened and led outside by a figure in a monstrous rabbit costume, who introduces himself as "Frank" and tells him the world will end in 28 days, 6 hours, 42 minutes, and 12 seconds. At dawn, Donnie returns home to find a jet engine has crashed into his bedroom. His older sister, Elizabeth, informs him the FAA investigators do not know where it came from.
Donnie tells his psychotherapist, Dr. Thurman, about his continuing visits from Frank. Acting under Frank's influence, he floods his school by damaging a water main. He also begins dating new student Gretchen Ross, who has moved to town with her mother under a new identity to escape her violent stepfather. Gym teacher Kitty Farmer blames the flooding on the influence of the short story "The Destructors", assigned by dedicated English teacher Karen Pomeroy, and begins teaching attitude lessons taken from motivational speaker Jim Cunningham. Donnie rebels against these motivational lessons, leading to friction between Kitty and Donnie's mother Rose.
Donnie asks his science teacher, Dr. Kenneth Monnitoff, about time travel after Frank brings up the topic, and is given the book The Philosophy of Time Travel, written by Roberta Sparrow, a former science teacher at the school who is now a seemingly senile old woman.
Dr. Thurman tells Donnie's parents that he is detached from reality, and that his visions of Frank are "daylight hallucinations", symptomatic of paranoid schizophrenia. Donnie disrupts a speech being given by Jim Cunningham by insulting him in front of the student body, then burns down Cunningham's house on instructions from Frank. When police find evidence of a child pornography operation in the house's remains, Cunningham is arrested. During a hypnotherapy session, Donnie confesses his crimes to Dr. Thurman and says that Frank will soon kill someone.
Rose agrees to replace Kitty as chaperone for her daughter Samantha's dance troupe in Los Angeles, so Kitty can testify in Cunningham's defense; with her husband Eddie in New York on business, her older children are home alone.
Donnie and Elizabeth take the opportunity to throw a Halloween party to celebrate her acceptance to Harvard. Gretchen arrives, distraught that her mother has disappeared. Realizing that only hours remain before Frank's prophesied end of the world, Donnie takes Gretchen and two friends to seek Roberta Sparrow at her house. They are attacked by two school bullies, Seth and Ricky, who are attempting to rob Sparrow's house, and the fight spills into the street. An oncoming Pontiac Trans Am car swerves to avoid Sparrow, who went for her daily walk to check her mailbox, but runs over Gretchen, killing her. The driver turns out to be Elizabeth's boyfriend Frank Anderson, wearing the same rabbit costume as the Frank of Donnie's visions. Donnie shoots him in his eye with his father's gun.
As a vortex forms in dark clouds above his house, Donnie drives into the hills and watches as an airplane descends from above. The plane, carrying Rose and the dance troupe, is wrenched violently as one of its engines detaches and falls into the vortex. Events of the previous 28 days recapitulate in reverse order and action, until Donnie finds himself in bed back in the early hours of October 2. As he lays in his bed, waiting and laughing, the jet engine crashes through his room, killing him. Others with whom Donnie had interacted in the 28 days awaken, some looking disturbed. Gretchen rides by Donnie's house and learns of his death from a neighborhood boy, David, but says she did not know him. Gretchen and Rose exchange a glance and wave as if they know one another, but cannot remember from where.

Question: Who tells Gretchen about Donnie's death?
Answer: David