Marriage: 23 year marriage, MRP for maybe 2 years, but never really too beta previously, never a dead bedroom, wife remains attractive, child now 18. Edit: 2 years ago lost our other son [many posters suggesting probably correctly this tragedy significantly alters this situation]

Problem: Last week, wife mentions she had lunch with another guy who she met at work, says they have lunch every now and again to discuss industry and catch up. I recognize this right away as preliminary hamstering and ass-covering. So I check the text messages (which I normally never bother checking), sure enough almost daily text messages, including talk of "stuff I can only comfortably discuss with you" and "I love our lunches," and "I adore our time together". But I believe nothing physical has happened, they have sent each other a single picture, him out hiking by himself (posing) and she sent one that included our son. It probably fits the definition of emotional affair so far, nothing explicit in the text log and no obvious deletion gaps. Were I to directly confront her with it, she would certainly trickle truth and hamster "We're just friends" and probably move to "Why are you so jealous I can't have any friends." and of course go nowhere.

RP specifically advises against Mate Guarding, which I agree with, until the behavior becomes a real problem. If this were physical cheating, even direct flirting by text or explicit picture sharing, it would be clear and I would obviously address. But right now it's right on the line of deniability.

The MPR approach to this, from MMSL, is to gather all the evidence, fill out the divorce paperwork, and have a nuclear decision session -- instructing her to either cut off all contact with this guy while I watch or I will submit the divorce paperwork to the court and have her served.

Am I going nuclear too soon?

The only other option I see is to ignore (and continue monitoring while it inevitably escalates) and continue to work on myself. If I ignore, I feel like I am setting myself up to become cuckolded, but if I divorce her over this it seems perhaps petty and she will feel fully justified, certainly will look pretty bad in the social world, not that I care much about that, but may even push her to press harder on the divorce rape button. Any halfway solution like telling her I found her texts and they bother me will result in denial, no progress, and me coming off as beta/butthurt.

There are other troubles in the marriage, so divorce would not come completely unexpected, but she would almost certainly be quite surprised to find this to be the final straw. Also, I am comfortable moving forward with divorce, child is legal adult now (thank god), but all things being equal I think the marriage could be made to work and eventually be pretty good, and I really would hate to burden our newly-adult son with divorced parents.

I am requesting advice.

TLDR; Wife texting other man constantly, super friendly but not yet physical, considering nuclear option to stop it before progressing.