In the film Dog Days, Madison is first seen driving a convertible with Heather to pick up Holly Hills after school. Heather tells Holly to get in the car, preventing Holly from signing Greg Heffley's yearbook. Rodrick Heffley tries to flirt with Heather, but his foot ends up getting run over by her.
Madison and Heather later meet up with Holly while she is teaching Greg tennis at the Plainview country club. Heather tells her that they are going home, as there is a problem with her Sweet 16 birthday party, forcing Holly to leave Greg.
Madison and Heather meet Holly again after Holly finds out that Greg is not a member of the country club. While Greg tries to explain himself to Holly, Heather rudely interrupts him and panickingly tells her that the DJ for her Sweet 16 has canceled and that there will be no music for the party. Greg suggests she hires Loded Diper. Heather at first does not seem to want to do so, but Madison convinces her otherwise by telling her that she will be the first in her school to have a live band play at her party. Heather then reluctantly hires the band.
At Heather's Sweet 16 party, Madison is with Heather when Heather slaps Rowley Jefferson's hand for taking a strawberry from the table. Heather reminds Greg and Rowley that they are roadies for Loded Diper and, like other employees, are not allowed to eat anything at the party. Holly comes up and defends the both of them, telling her that they are her friends. Heather gets upset at Holly, accusing her of being selfish, to which Madison agrees.
Later into the party, Loded Diper performs a rock version of the song Baby, to which Rodrick is the singer. Rodrick performs the song horribly, angering Heather. Madison watches as Rodrick accidentally knocks over the ice sculpture of Heather, causing her to snap in anger, Heather grabs a microphone stand and tries to hit Rodrick with it. As Heather carries the stand over to Rodrick to hit him, Madison follows. Heather tries to hit Rodrick by swinging the stand but Rodrick ducks and Heather hits the chocolate fountain, causing it to spray on Heather and Madison. Madsion, despite agreeing earlier about no roadies being allowed to eat anything, still allows Rowley to eat a strawberry and dip it on her (which hints that she doesn't agree with anything Heather says, but still does so so she can be Heathers friend). Madison's fate is left unknown after this, though it's possible that, judging by her attitude, that she and Heather parted ways, making Heather a loner.
Like Heather, Madison is a stereotypical valley girl, and as such, is narcissistic, spoiled and selfish, as shown when she runs over Rodrick's foot when he tries to flirt with Heather. She is portrayed as the sidekick best friend of Heather, following her wherever she goes. She does whatever Heather tells her to do and often repeats what Heather is saying. However, she is shown to be less mean than Heather, being open to Greg's suggestion of hiring Loded Diper, as well as letting Rowley dip chocolate on her.
- Although Madison does not appear in any books, her animated version can be seen in the credits, when she is laying on the beach with Heather, as Greg tries to impress them by showing off his muscles.
- She and Heather go to Crossland High School, as she says that Heather would be the first at their school to have a real band.
- She might be a Country Club member.
- It's possible Madison may not agree with anything Heather says and does, but only does what Heather tells her so she can be friends with her. This can be seen when Greg suggests hiring Loded Diper, she is open to it despite Heather's rejection, and despite her and Heather not allowing Greg or Rowley to eat the strawberries, she still lets Rowley eat one and allowed him to dip chocolate on her.
- Madison’s fate is unknown after Heather’s sweet sixteen, but it’s possible her and Heather parted ways.