Eric, I don't think you can do anything at all about her reactions to your breaking off the relationship. I think I would just cleanly tell her you do not wish to continue the relationship and that you will not be writing her anymore or reading her emails.

I'm sure, if the woman has troubles with her husband and tells you about them, she enjoys talking with you (because it's someone to talk to, unlike you not needing to talk to her about your wife there's a problem there in her relationship that she's talking to you about instead of her DH) and wishes you'd talk about your relationship. That you don't probably feels "unfair"/one sided to her. Too, that your wife called her a couple times (what was THAT about?) changes the mix.

I don't think you can/should say much about your wife as she and the woman have their "own" relationship since your wife called and talked with the woman so there's no way that you can "judge" her opinions of your wife. If it would make you feel better and/or she demands a "reason" for ending the relationship you can say you did not appreciate the comments she made about your wife but your wife calling and talking to her was similar to her talking to you about her husband so you're kind of "in the middle" now without a leg to stand on.

I'm glad you are ending the relationship, it does not sound like it is going in a positive, healthy direction anymore. While I think a man and woman who are not married to one another can have a friendship such as yours and the woman's, one has to decide if it is worth it if one's spouse does not like it.