The Dentist

Dr. Alan Feinstone is a successful dentist. However, everything changes on the day of his wedding anniversary, when he discovers his wife Brooke is cheating on him with the poolman, Matt. After they finish, Alan retrieves his pistol and follows Matt in his car. He is led to Paula Roberts's house, a friend of Brooke's. Alan invents a story about a surprise party for Brooke and watches Paula invite Matt inside. Paula's dog attacks Alan, and he shoots it in self-defense. After returning to his car, he drives to work.
At his dental practice, Alan's first appointment goes poorly when he hallucinates a child has rotten teeth and accidentally stabs him. As Detective Gibbs investigates the death of Paula's dog, Alan sees his second patient, April Reign, a beauty queen. Alan hallucinates she is his wife, and, while she is unconscious, takes off her pantyhose and fondles her before choking her. As she wakes, Alan snaps out of it and hides her pantyhose. Alan tells his manager, Steve Landers, she is still dizzy from nitrous oxide. When Steve realizes what really happened, he returns, punches Alan, and threatens a lawsuit. Alan ends the day early and sends his staff and patients home, including Sarah, a teenager who wants to have her braces removed.
Later that night, Brooke meets Alan at a new opera-themed room at his practice. After sedating her under the premise of cleaning her teeth, he pulls out her teeth and cuts off her tongue. Detective Gibbs and his partner Detective Sunshine arrive at Alan's house the next morning to ask him questions. After the policemen leave, Matt discovers Brooke, who is still alive but sedated. Alan stabs Matt to death.
Sarah and Paula are waiting for Alan at his practice. Alan sees Paula first, much to Sarah's disappointment. When Paula's conversation turns to how good a job Matt does for her, Alan overly-aggressively drills into her tooth, destroying it. His assistant, Jessica, questions what he is doing, and he snaps out of it. Alan asks Jessica to finish for him, but after he discovers she has sent Paula home, he fires Jessica. When Jessica pulls out April's pantyhose and threatens to expose him, Alan kills her.
At the police station, Detective Sunshine discovers that the bullet pulled from Paula's dog's only matches one gun in the area: Alan's. He and Detective Gibbs drive to the Feinstone house to question him further. Near the pool, they discover Matt's body. They quickly break into the house and find the mutilated Brooke, tied to the bed but still alive. Meanwhile, IRS agent Marvin Goldblum, using Alan's tax problems as leverage, extorts a free dental exam and a payout. Instead, Alan tortures him. Later, Alan's other dental assistant, Karen, finds Marvin still in the dental chair. Alan kills her by inserting a needle full of air into her jugular vein.
After Alan removes Sarah's braces, he imagines her teeth rotting. He pulls his gun, but she escapes and hides in one of the dental rooms, where she finds the blood-soaked Marvin, who attacks Alan. When Alan recaptures her, Sarah hysterically promises to brush her teeth three times a day and to never eat candy. Satisfied, Alan leaves. The two detectives arrive, but they are too late.
They follow Alan to a university, where he teaches dentistry classes. There, Alan maniacally instructs all of his students to pull all of the teeth out of all their patients. As he hallucinates and shoots a dental student that he mistakes for Matt, the detectives burst into the room, but Alan uses a hostage to escape. Eventually, he wanders into an auditorium where an opera singer is practicing. Enchanted, he watches her from behind. When he reaches out to touch her, she transforms into Brooke, who laughs at him. Defeated, he falls to his knees and is arrested by the detectives.
Alan, now in a psychiatric hospital, is carted off to his regular dental appointment. The dentist working on him is revealed to be his toothless wife Brooke, who works violently on his mouth.

Question: Who is Brooke having an affair with?
Answer: Matt