Stealing Harvard

John Plummer (Jason Lee) is engaged to Elaine Warner (Leslie Mann), and intends to use his life savings of $30,000 to put a down payment on a house. He works for Elaine's father, Mr. Warner (Dennis Farina), who dislikes John. Simultaneously, John's niece Noreen (Tammy Blanchard), daughter of sister Patty (Megan Mullally), is accepted to Harvard University, but needs an additional $30,000 on top of her grants and scholarships. Noreen shows John an old videotape where he promised to pay for Noreen's college. John now has a moral and financial dilemma â disappoint his fiancÃ©e or disappoint his niece and ruin her chance at escaping poverty.
John confides in his friend Walter "Duff" Duffy (Tom Green), a landscaper. He convinces John to steal from one of his rich clients, who keeps large amounts of cash in an unlocked safe. The pair set off to steal the cash, but Duff runs away when lights come on in the home, leaving John to get caught by Emmett Cook (Richard Jenkins). Cook forces John to cross-dress and role-play the part of Cook's late wife as the two men lie in bed and "spoon". Eventually, after taking an incriminating photograph of John, Cook releases him. As he is leaving, Mr. Warner rides by and takes note of John's panicked behavior, believing that he has caught John in an affair.
Further capers ensue as John and Duff try to rob a liquor store and later attempt to con a drug lord out of $30,000 by concocting a phony story about running an ecstasy ring. A police detective (John C. McGinley) is on to John and Duff, but never has enough evidence to actually pin any of the crimes on them. Meanwhile, Mr. Warner breaks into Cook's residence in order to get evidence against John, and once Cook catches him, he is forced to "spoon" as well. Before leaving, Warner finds the photo of John from the album, which he then gives to Elaine.
John is forced to confess everything to Elaine, who is not upset and admires the lengths he was willing to go to in order to please her and send his niece to Harvard. Elaine then confides in John that her father keeps a great deal of money at his business, and that it would be easy for them to steal it. John, Elaine, and Duff set out to rob the business in the night. Unfortunately, Mr. Warner had hid his dog Rex inside the vault. Rex latches on to Duff and does not let go. Just as John and Elaine find the money, Mr. Warner tries to attack them but he is caught by the detective who mistakes him for a burglar. John and Elaine escape to Duff's van. The police arrive and the gang unsuccessfully tries to get away. They are all taken into custody by the detective and facing a series of charges. John feels doomed, until the judge in charge of his arraignment turns out to be the gun-toting Emmett Cook.
Upon their mutual recognition, John flashes a written message to Cook, threatening to expose the judge's fetish; upon reading the note, Cook quickly dismisses all charges against John. Finally, Duff comes through as best he can and gives John his life savings, $1,000, which John bets on a long-shot horse which wins and which paid 30 to 1. John and Elaine are married, Noreen goes off to college, and, in the final scene, John is left to ponder how loser Duff could possibly accumulate $1,000 â the last scene shows Duff offering to "spoon" with Cook for $1,000.

Question: What forces Duff to run away from the home they are robbing?
Answer: The lights come on