For me the changed passwords and Friends on FB would be a big problem. Another man is being seductive with your W with her sending nude pictures back and she has a problem with you knowing about it? Really? Unfriend was a must for me. They crossed a line that can't be undone. Loss of "friendship" is the consequence in my book. It's you or him, not both. "Sorry W, you went too far now you have to choose."
That is the right thing to do from the beginning. Don't believe what she says or threatens you with. You have to take charge of this, set your boundaries that this is unacceptable and follow through. At least she will respect you for this. Sorry to hear. I've been there and made all the mistakes. The thing is you cannot really TALK to her right now. She is lying to you and probably also to herself. This cannot be solved with words right now. Make yourself clear, not more and then work on yourself!
Me 32 (German) Wife 28 T 3yrs M 2yrs Moved to US for W No kids BD 6/2014 In house separation Confirmed EA 1/2015 (ongoing since BD) OM not ready Real D talk started 1/27/15