Re: new here: proof of impending PA, do I intervene?
I would posit an alternative possibility. Just because your wife has the body of an adult DO NOT assume her mind is mature. Intellectual growth and maturity is not as easily determined as is physical growth. The indicators are much less visible. In order for anyone to make the "right" decision" they must have the capability. It is difficult for those of us to comprehend that are cognizant enough to see clear divisions between right and wrong.
I believe that you may have jumped the gun and may be reading too much into your initial discovery. It is entirely possible that she does "love" you in her way, the way she is capable of. This may not fully coincide with your way however. We all process information differently and her texting, in her mind, may be benign whereas in your mind it is cheating. I would advise you to confront her with your information and to ask her to explain how she feels this is appropriate. Then explain to her how you see it and, based on her intellectual ability, she will either understand and modify her behavior or she will try to lie, become angered and accuse you of being controlling and overbearing.
If the former occurs then you are indeed fortunate however, if the latter occurs then you must now make a decision. You will now know that she cannot be trusted to make right decisions that are good for the marriage and your family and you will then need to make them for her or dissolve the union. Lack of processing ability will always lead to erroneous conclusions. Therefore, to leave her decisions up to her would be to invite disaster into your marriage.
I once was as you are now and did not understand that my wife was incapable of making the right decision. I assumed she was an adult. I was proven wrong. Knowing what I know now I would have intervened and made decisions for her, explaining why it was necessary to do so. It is easy to recognize the barrel of a gun and understand the danger it poses, it is more difficult to recognize less apparent dangers, to do so requires more cognitive ability than she possesses. The OM is a loaded gun to your marriage but to her he represents no danger. It is up to you to show her the inherent danger and thereby protect your marriage.
Think of it this way, as if it were your daughter about to go on a trip that you knew would endanger her. You see the peril, she does not, would you let her go? Or would you try to save her from herself? Many people do things that they sorely regret later in life. Do not let this be one of those times for your W. Communicate with her, explain the situation to her, show her the error of her ways and gauge her response. She will either come to appreciate you for "saving" her or she will fight you, ignore you and proceed on in her childish way, in which case she is most likely not "savable". You can say however, at the end of the day, that you did what you could to help her.
Peace and long life