Thinner

Billy Halleck (Robert John Burke) is an obese, upper-class lawyer living with his wife Heidi (Lucinda Jenney) and their daughter Linda (Bethany Joy Lenz). Billy recently defended an underworld crime boss named Richie "The Hammer" Ginelli (Joe Mantegna) in court. The town he lives in is hosting a carnival, run by gypsies that the townspeople hold obvious prejudice against.
One night, while Billy is driving, Heidi performs a blowjob on him. Distracted, Billy accidentally runs over Suzanne Lempke (Irma St. Paule), an elderly gypsy woman, as she leaves a local pharmacy. Since Judge Cary Rossington (John Horton) is a personal friend of his, he soft-pedals the case, and no charges are filed against him. Outraged by the injustice, Suzanne's 108-year-old father, Tadzu Lempke (Michael Constantine) curses Billy by touching his face and saying the word "thinner". Billy begins to lose weight rapidly, regardless of how much he eats. Heidi, fearing the weight loss may be due to cancer, brings in Dr. Mike Houston (Sam Freed), with whom Billy suspects his wife is having an affair.
It is revealed that the man also cursed Judge Cary and Police Chief Duncan Hopely (Daniel von Bargen), who helped him avoid punishment in court. Cary, whose curse was "Lizard", is now growing scales all over his body. Duncan, whose curse was "leper" becomes mutated with large boils and eventually commits suicide. Billy continues losing weight and calculates that he only has a few weeks to live. Billy looks for the gypsy carnival, to get Lempke to remove the curse, but they have gone.
When Heidi continually mentions Dr. Houston, who is repeatedly visiting their home, Billy is convinced of her affair. He lashes out at her and blames her for the accident. He finds the gypsy camp and tries to reason with Lempke, but instead angers him into increasing the curse's effects. Gina, Lempke's great-granddaughter (Kari Wuhrer), uses her slingshot to shoot a large ball bearing which goes directly through Billy's hand. Billy is forced to leave, but not before furiously vowing revenge against Lempke and his gypsies. Billy then enlists Richie Ginelli's help. Richie pays Frank Spurton (Josh Holland), a local man, to track down the gypsy camp. He then kills their dogs, and leaves a message demanding Billy's curse be removed. In retaliation, Gina and her husband Gabe kill Frank. Richie brings a gun to the camp and begins shooting, then pushes Gabe out into the open to be accidentally shot and killed by his fellow gypsies. The next day, Richie and Billy kidnap Gina and force her to tell Lempke to meet them.
At the meeting, Lempke finds Billy emaciated and near death. To prevent further attacks on his people, he decides to lift the curse. Lempke explains that the curse cannot be removed, only transferred to another person. Chanting a spell, he mixes Billy's blood into a strawberry pie. Lempke states that after being consumed by an unsuspecting person, the pie causes painful but rapid death, and the curse will be lifted. He urges Billy to eat the pie himself and die with dignity, but Billy ignores him. He calls Linda, telling her to spend the night at her friend's house, so that he and Heidi have the evening to themselves. He arrives home and presents to Heidi the strawberry pie, which happens to be her favorite flavor. She delightedly eats a piece, while Billy heads to bed, exhausted.
The next morning, Billy finds Heidi's desiccated corpse next to him. He is gleeful to be free of the curse and of his disloyal wife. However, when he goes downstairs, he finds to his horror that Linda, who came home after he went to bed, had eaten some of the pie for breakfast. Wracked with guilt, he prepares to eat the rest of the pie. However, Billy is interrupted by Dr. Houston who is at the door. Seeing Billy, the doctor grows uncomfortable and struggles to explain his early and unannounced presence. Billy invites Houston in for a piece of pie, and closes the door with a smirk.

Question: Who is Heidi having an affair with?
Answer: Dr. Houston