I honestly believe the wayward wife thinks the H is such a dufus that she can tell him anything and he'll buy it as the truth. That is one reason I think the old excuse of "they are only friends" is used so much.
Yes, she is playing you, completely, about the sleepover.
Please don't be quick to tell her anything pertaining to the legal system, child custody, etc. You have not seen bad until you see the wrath of a WW. And besides, you don't want to show all of your cards. Don't know where you live, but don't wait too long before interviewing a lawyer. Ask questions to know your legal rights as a father and what you need todo to protect yourself. Make sure you have a lawyer who is not shy about fighting for father's rights.
I am glad you mentioned that book, Women's Infidelity. I was trying to remember that title recently, without having to dig it up. I read the book during the time I was coming out of my waywardness, and was probably one of the best things for me at that time (besides the DB board). Kind of put the fear of God into me, so to speak.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!