How far should I take things with my ex's good friend?

Throwaway account for obvious reasons....

A few months ago, I posted an ad on Craigslist looking for someone to exchange sexy emails with. I had a huge number of responses, but there was one in particular that stood out from the others. Our quick little emails quickly evolved into full-blown erotic stories. We would take turns creating scenarios, trading off chapters. There was very little personal interaction outside of the stories, but, on a purely mental level, we obviously turned each other on.

A couple of days ago, my anonymous email partner asked if I wanted to start getting a little more personal and maybe learn some more about each other. He said that he was perfectly happy to keep things at the level that we were already at and there was no pressure. As I had gotten extremely curious to learn more about the man behind the words, I told him that I'd be happy to throw some more personal info into the ring and see what happened. At the end of my response, I threw in a PS stating that I was a bit worried that this was someone that I already knew. I signed my real name (which is rather unusual).

His response was that he only knew one other person with that name and it would be a helluva a coincidence if I happened to be the same woman by that name who had divorced his good buddy, ________. When I read that, my jaw dropped. Yep, I was his good friend's ex-wife.

So now I'm trying to figure out what to do. The emails that have been exchanged between ex's friend and I have turned me on on a level that has never happened before. We discussed it and decided that we were both comfortable with things where they currently were, but if anything changed, we'd need to talk it over thoroughly.

My ex and I have been divorced for about 7 years and he's happily remarried. He and I remain friends (but not good friends). He was never a jealous man and we actually had an open marriage where we were allowed to sleep with other people. (No, I never slept with his friend.)

I should also add that I have PTSD and meeting new people can be extremely difficult for me. (It's part of why I advertised to just email with someone instead of meeting in person) The anonymity provided by the internet has allowed me to explore and experiment while still feeling safe. Because my email partner is someone that I know and can trust, it is easy for me to think about transferring some of these fantasies to real life.

I guess I have a couple of questions for 2X. How far is too far? (Is there a "too far"?) At what point, if any, do we need to discuss this situation with my ex? We've discussed exchanging pictures, but I'm still trying to wrap my mind around the fact that he is who he is, so haven't done that yet. Anyone have any feedback for me?