The Hills Run Red

Tyler is obsessed with the horror film The Hills Run Red, considered the scariest movie ever made, with the deranged serial killer Babyface in the lead role. However, the film's director, Wilson Wyler Concannon, disappeared years ago and there is no known copy of the movie. Tyler's obsession with the film leads him to neglect his girlfriend Serena. When Tyler discovers that Concannon's daughter Alexa works in a night club as a stripper, he decides to meet her and ask about the lost film. He visits Alexa and asks her about the project. As she gives him a nude lap dance, the audience sees Serena cheating on Tyler with their best friend Lalo. Alexa informs Tyler that the movie might be in her father's home in the woods.
Tyler drives out to Concannon's home, accompanied by Serena, Lalo, and Alexa. On the way, they are bound and attacked by rednecks, who threaten to rape Alexa and Serena. Babyface appears and kills the attackers before chasing the teens into the woods, and then disappears into the woods. Tyler breaks into the house and finds a red room that contains many film reels hanging from the ceiling. When Tyler hears pain-filled screams, he finds Alexa tied to a bed and releases her. The two leave the house and find Serena, but Babyface breaks through the door, brutally throws Tyler into a wall and injures Serena. Alexa is slammed against the wall by Babyface and says "fetch," telling him to run after Serena. Tyler drops unconscious as Alexa approaches him, smiling. Serena hides under the car parked in front of the house. Babyface enters a barn as Serena quietly moves to an adjacent building. Serena is shocked to find bloodied bodies in the room, which turns out to be a smoke house. When Babyface goes inside the smoke house, he doesn't find her and leaves. Serena emerges from a drum that was used to store blood and changes clothes. However, when she leaves the smoke house, Babyface jumps down at her from the roof.
Tyler wakes and finds himself tied to a wheelchair. On the shelves behind him are many film reels, all titled The Hills Run Red. Tyler is shocked when Concannon, the missing director of the film, appears. Concannon tells him the secret behind the scares in his film, and plays a video of twenty years before, when the film was in shooting. Concannon is dissatisfied with the actor portraying Babyface, and angrily explains to him how to properly make a scene feel scary. He takes Babyface's costume and axe, and violently axes the actor to death. It is then that the reason why screenings were canceled and all actors vanished is revealed: the film was considered too violent, and all the death scenes in it were real. He also tells Tyler Alexa was only thirteen years old when she "had him", indicating Babyface is a child born of incest between father and daughter.
Babyface is called away by Wilson and returns to the barn, leaving Serena alone. She breaks free from her binds and successfully escapes the room. Alexa stabs Lalo in the chest, killing him. Concannon tells his daughter he's the only director of the family and shoots her. Babyface, angered by Alexa's apparent death, turns on Concannon and fights him. Concannon screams out that he is Babyface's father. Tyler takes a camera and encourages Babyface to kill Concannon â which he does. Babyface then turns on Tyler but before he can kill him, Serena stabs Babyface through the back with a long iron staff, apparently killing him. However, Serena is hit by a shovel, and Alexa, still alive, hits Tyler as well, knocking him unconscious. Alexa then ties Tyler up and forces him to watch her cut of The Hills Run Red, driving him insane.
In a post-credits scene, Serena, still Alexa's prisoner, is several months pregnant. Alexa sings a lullaby to Serina's unborn baby, and the end credits continue to roll as Serena screams.

Question: WHO IS SERENA CHEATING WITH ON TYLER?
Answer: LALO.