Does it matter if the EA was reciprocal? If your W has formed an emotional attachment to the OM, I would say the point of reciprocity is fairly moot. Once the hormones flood their head, they head off to lala land. I'm not saying that you shouldn't continue to try and piece together whatever evidence that they have been careless enough to let slip, but it seems pretty clear that she has already checked out with you, and in with him.
My sitch is very similar to yours: OM was just a friend, she would never do anything, etc. etc. I was stupid enough to let them go to a motel at the beach together, a cabin by the lake, he stayed at my house every day for 10 days while I was at work, etc. etc.
Sorry to break it to you, but an EA is an A, sexual or not.
Me40 WAW37 M18 T20 S18,14 D13 EA Bomb 6/08 Sep 11/20/08 Ret 08/09 Sep/Filed 11/09