Let me lift this up to you.
Why deal in messy then?
- You have come away from a cheating affair. Burden lifted. (good)
- You have told your wife everything you had been feeling but not saying for so many years... feeling disconnected, restricted, frustrated. That's a burden lifted. (Good.)
- You are enjoying renewed connection with your spouse and greater understanding. Joys shared. (Good)
This is you looking out for your own best healths how?
(Mental health, emotional health, physical health, spiritual health.)
I feel like I'm enabling M to cheat, even though the cheating in this case is just friendly conversation.
You are. You are aware of this agreement existing and cannot "unknow" what you already know. What is your behavior here? Clean ethics or not? Not.
If her agreement with her husband is "no communication at all" with you, that you communicate and have coffee dates is helping her to break one of her relationship agreements. (Whether or not the agreement is rational and reasonable is another story, but she would have to go to him to renegotiate that limit. The agreement is with him not you. )
You could encourage her to finish with the old thing (no communication agreement changing to a new agreement that allows her to communicate with you) before starting a new thing (friendship with you.) Demonstrate that YOU are worth being treated nicely and worth the bother of straightening up her ethics for.
You could let her know you are open to friendship, but don't want any truck with "messy" or "weird" and like your OWN boundaries and limits respected. If she's needing aid to LEAVE a bad marriage, that is one thing. But skulking in the shadows all "cheaty" again? No, thanks. You are done skulking.
You could tell her it was hard work with your spouse, but you did it anyway and came out the other side. You could encourage her to do same with her spouse -- break it clean or work it out clean. But clean,
hot ethics. Not dirty ones, please.
You could tell her you don't like feeling yucky, so you choose not to engage in behavior that brings you this yucky feeling.
Could tell your ex-cheating partner where you stand, and where your updated boundaries are. Then let her sort her life and come to you for friendship once she's gotten to the place where she too can keep her own nose clean. Then you don't have to feel yucky about being an enabler. It bothers you on some level or you would not be posting.
So... let's just move it forward then! Play ethics ball right.
Get it back in bounds. Everyone holds their own baggage.
My 2 cents,