I'll tell my story, which has a slightly different perspective.
My husband has been with his GF for almost 2 years. She is cheating on her boyfriend, has other partners who cheat on their partners with her, and has cheated on my husband.
The situtation is not exactly the same as yours - she lives in another city, our social circles don't overlap, I almost never see her, and don't know any of her other partners.
So I'll just focus on the "I don't know who she is anymore, I thought she was an honorable person" part of your situation.
I have struggled very very hard with this, and am sometimes still struggling. It has been a major life lesson for me to get to the point where I understand that it is possible to love, honor and respect a person who loves someone I cannot love, honor or respect. And there are moments, even 2 years in, when I still get that feeling "Who are you?"
My husband is hs own person. He chooses his relationships. He chooses to be with her for reasons that are beyond my grasp. I try not to judge him for it, and focus on how he treats me (and there have been bumps in the road there, as well.. some broken rules and trust, little lies, etc., mostly in the beginning under the influence of NRE) and how he behaves towards me.
There's nothing I can tell him to do or not do when it comes to his one on one relationship with her. I can say what I do not want for myself, or for my life. Like, that I don;t want to socialize with her. That I don't ever want to be put in the position where I would have to lie for her. Certain things I don't like her to do when she is at our house (when I am not there).
I am not saying that this is a point you should get to. Certainly the situation where you know C.'s wife, and they work together, very much complicates things. I just wanted to point out that it is possible to have a happy poly relationship with someone who is in a relationship you don't like / approve. That is not a common viewpoint on these boards I think. And it's not ideal, but it's also not impossible.
early forties, straight.