Just my 2 cents, but my W and I made zero progress until I confronted her about her EA. In my case I had evidence- her secret email account. Once it was out in the open she finally opened up to me. A word of caution- have hard evidence!! My wife tried to deny until she knew I had her cold. For a girl that was always so truthful she was a very good, and adamant, liar (which really surprised me). Had I not seen it with my own eyes I'm sure she would have convinced me I was crazy.
Again, my opinion, but why let her get closer to this guy while you pretend it isn't happening? You've been doing that (in essence) since July- has it helped? Now that you KNOW you should get hard evidence and confront her (again, just my opinion). You might be able to stop it from becoming a PA if it has not already gone there.
All is fair in love and war. I wasn't going to give the OM a single advantage, I suggest you do not either. I still have not told the OMs wife about what her husband was up to (didn't want their two kids drawn into the fire), but I still have that card if I need it (if he does not steer clear of my W).
Best of luck!! I pray your marriage survives this storm.
