Been looking for this story. Not sure where I read it.
A son seduces his neglected and reluctant mother, hinting at his father's infidelities. Eventually, he reveals that for years, dad had been screwing the wife of their couple friends. He even takes her out near where they used to camp, so that he can have sex with her, while watching her husband cheat. By the last chapter, mom is an enthusiastic participant, loving not only the sex, but alos the revenge. She even arranges for her son to also seduce his father's mistress.
