I wouldn't contact his parents until AFTER you figure he has told them. It is his family. And I understand why/what you want to do, but its not your place to be the one that ends up letting them know that things are over or almost over. I don't think any good would come of that. You can let them know when the time is right...and if you let them know and they start asking the questions, then you are letting your H off the hook. I doubt it will be easy for him to let his parents know, so let him do it, let him deal with the consequences of this actions, let him deal with their initial reactions and emotions and questions...not you.
I'm no expert at this Infidelity stuff, but I am a pretty good student of human nature. So I feel entirely confident in saying that "He will lie to them about the nature of his relationship with OW."
