American Reunion

Thirteen years after graduating from high school, Jim Levenstein (Jason Biggs), Chris "Oz" Ostreicher (Chris Klein), Kevin Myers (Thomas Ian Nicholas), Paul Finch (Eddie Kaye Thomas), and Steve Stifler (Seann William Scott) are well-established in their lives and careers. Jim is still married to Michelle (Alyson Hannigan) and now has a two-year-old son, Evan. Since the birth of their son, Jim and Michelle's sex life has deteriorated. Oz is an NFL sportscaster living in Los Angeles with his supermodel girlfriend Mia (Katrina Bowden). Kevin is married to Ellie and works from home as an architect. Finch tells his friends that he has been travelling the world, and still searching for his one true love. Stifler works as a temp at an investment firm, where he is also the victim of humiliating verbal abuse by his cold-hearted employer.
Former classmate John (John Cho), one half of the 'MILF' duo, organizes a Class of 1999 high school reunion in East Great Falls. Jim and Michelle return to Jim's old home, where his father Noah (Eugene Levy) is now a widower. Jim encounters his neighbor Kara (Ali Cobrin), whom he used to babysit and who is soon to turn 18. Jim meets Oz, Kevin, and Finch at a bar, where they meet Selena Vega (Dania Ramirez), Michelle's best friend from band. Stifler appears unexpectedly, and joins them for weekend activities.
The next day, the group goes to the beach. Oz meets his high school girlfriend, Heather (Mena Suvari), who is dating a cardiologist named Ron (Jay Harrington), and Kevin reconnects with Vicky (Tara Reid). The guys have an altercation with Kara's boyfriend, A.J., (Chuck Hittinger) and his friends, which ends with Stifler defecating in their beer cooler and destroying their jet skis. That night, most of the group go to the falls and find a high school party celebrating Kara's birthday. Finch and Selena reconnect, and they fall in love. Kara gets drunk; Jim drives her home, and she tries to seduce him. They are discovered by John, who simply mistakes Kara for Michelle. Oz, Finch, and Stifler help Jim return Kara to her parents' home, but A.J. spots them. Kevin wakes up hungover next to Vicky and assumes they had sex.
Jim and Michelle attend a party thrown by Stifler, hoping to recreate their prom night, and bring Noah along. Noah gets drunk and encounters Stifler's mother Jeanine (Jennifer Coolidge) for the first time. Kevin confronts Vicky about the night before, but she insists there was no sex. Mia takes ecstasy, and Ron humiliates Oz. A newly sympathetic Heather comforts Oz, and they reconcile. They are interrupted by Mia, who starts a fight with Heather. Stifler performs oral sex on a former classmate named Loni hoping she would return the favour, only to strike out. By this point in the story, Jim has told Kara that he does not care about her. A fight ensues between A.J. and his gang against Jim and his friends, which is disrupted by police. But the police only arrest Finch for stealing a motorcycle, which Stifler finds amusing. Exasperated by Stifler's immaturity, the guys blast him, and finally admit that they did not want him to ruin things like he always does. Distraught, Stifler ends the party.
Mia leaves Oz, Stifler decides to skip the reunion, and Michelle goes to her grandmother's. At the reunion, Finch admits that he is an assistant manager at Staples and stole the motorcycle from his boss when he did not receive a raise that was promised. The boys apologize to Stifler at his place of work, and insist that he is their friend; without him, high school would not have been any fun. Stifler is uplifted by this and quits his job - but not before standing up to his patronizing boss - and attends the reunion. Kevin reconciles with Vicky. Finch makes amends with Selena for lying, and they have sex in the bathroom - but not before a premonition of something vague but bad happening, which is that Stifler is having sex with his mother, as revenge on him for having sex with Stifler's mother. Oz reunites with Heather (a result of Stifler beating up Ron), and Jim reconciles with Michelle. Jessica (Natasha Lyonne) reveals that she is a lesbian, and with Stifler's help, Sherman (Chris Owen) hooks up with Loni. Stifler is asked to be a party planner for a wedding for his former lacrosse teammates. He also meets Finch's mother Rachel (Rebecca De Mornay) who intrigues Stifler by telling him she wishes her son were more into sports. After Rachel makes a brazen sexual pass at Stifler using lacrosse equipment-related innuendos, Rachel and Stifler proceed to have sex on the lacrosse field, while Rachel loudly and passionately agrees with Stifler's proclamations about his manliness. John is reunited with his estranged buddy, Justin (Justin Isfeld) (MILF Guy #1) and they watch Stifler having sex with Finch's mom while chanting "MILF! MILF! MILF!".
The next morning, Jim and Kara share a tender moment while apologizing to each other for their behaviour. Oz plans to stay in town with Heather, Finch plans a trip with Selena to Europe, and Stifler drops subtle hints about sleeping with Finch's mom. Kevin proposes a pact for them to reunite once a year. They all agree and make a toast as the franchise's theme song ("Laid") plays. As the credits roll, Stifler says, "I fucked Finch's mom!" in a half-whisper, leaving Finch disgusted. In a mid-credits scene, Noah Levenstein receives sexual gratification from Stifler's Mom at a movie theatre.

Question: Who interrupts Heather and Oz's reconciliation?
Answer: Heather