I think that is it, midlife crisis (based on the very little I know about the situation and not knowing you or her). Definitely don't play the calm and understanding "everything will be okay honey, I still love you," chump role and threaten her (with divorce/leaving/taking the kid), and mean it. Or she will have the upper hand and will know she can get away with it. If you show her how pissed you are at her alleged actions, and what you're willing to do about it, it might put things in perspective for her.
At this point, I think it's more about your feelings, not hers, since you are the victim and basically didn't do anything wrong, unless there is more to the story that you aren't sharing that would cause her to stray.
Edit: and I can't remember who was telling me their similar story, or maybe it was another board story, but they're wife started doing the same thing, hanging out with the younger crowd at work, started going back out to clubs and drinking, while the husband was left at home with the kids wondering why his wife was acting out this way. Midlife crisis: Men buy expensive cars (or toys), I guess women need something else to make them feel like they are still young.