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She may be going through a mid life crisis but whatever is causing her behavior, it has to be addressed. She may think that she is in love at the moment. What happens would depend on both of you. It would be best to address this sort of a situation in couple's therapy. If she is not willing, then you'd still have to decide what you want to do about this. Have her move out, give her a time frame during which she is to decide what she really wants, etc.
Having an open communication would be important to figure out what may have led her to behaving this way. Was she expecting something from you/the marriage, how long has she felt this way prior to meeting this other man, what is her outlook of the future whether she leaves you and stays with him or decides to work on the marriage.
Once the "in love" stage is over, she may see things more objectively. You would have to think about what you want out of it and whether you'd be able to forgive and trust her fully after this incident.