Shecan say, “I’m sorry, he loved you so much, it was all me, I offered it up on a plate, he kept refusing but eventually I wore him down, I knew what I was doing, and can’t help going after other woman’s husbands, it wasn’t his fault, it was all me, he kept telling me over and over how guilty he felt, how much he loved you, I’m a shit human being, thank you for making me see sense, I’m so hurt by your text and devastated, but I’ll never do it again. It’s hard to answer your questions but to be very clear, he never wanted me, only you.”
Clearly she’s never going to say that. But I think that is along the lines of what the op is hoping to hear.
Even if she does say that, and I agree it's most unlikely, it's no help if OP still can't trust him not to do it again. She can't even trust it's true, OW might be lying to help him out. Her husband's faithfulness is the only one that matters, and if she can't trust him to speak or act in truth, it makes no difference what any random woman elsewhere might say or do.
There was a thread on here a while ago in which a former OW, who had an affair with a married man when she was much younger, received a letter from the wife essentially trying to bring her down with moral righteousness. Her reaction at the time had been "tell it to your husband" and to laugh at the wife with her friends for being pompous and superior. Now, many years later, she has a better understanding of why it was wrong and wouldn't do it again, but that was nothing to do with the letter, just her own age and maturity. She still thought "tell it to your husband"....