How would the son plausibly participate in helping his mom cheat. Even if dad was violent or cheating it seems unlikely.
Maybe the son starts off as an unwitting helper. He provides cover or an alibi. Eventually he figures out what she has been doing. If she is cheating with multiple partners then the son could decide that he wants to take advantage of the situation and tells her that she needs to do something for him if she wants him to keep her secret. She explains that she has an unnatural sexual appetite and she feels guilty but she needs to have more sex than her husband can provide, maybe he has a low libido or ED. Son decides to stand in for her and makes her promise not to cheat anymore so she has to come to him exclusively now. They start a torrid affair, having sex all the time and all over the house.
