I vote for contacting her, but give yourself time. Write the letter a couple ways. Write the screaming one that you won't send, first [use red ink, or a red font color if you do it on the computer, so you won't make a mistake and mail it by accident!]. Then give yourself a break for a couple hours or a couple days, and write the grown up one that you will send. In which you start off by apologizing to her, even if you don't feel apologetic, because no matter what he did or why, he had an existing obligation to her first, and you intruded on it... even though he initiated things. After all, you could have said, 'no, you're married.' So an apology for not saying that is a good place to start.
This will get her off the defensive, and you can then move forward from there to say that you know it was wrong to be involved with him, and that you have no intention of doing so again, and that you want her to be aware that he has been making efforts to reconnect with you on some level. You don't know what level, but you don't care, and you can tell her this, too. You know he is not trustworthy - look what he did to his own wife with you as an accomplice - and you wish to make amends to her, and you want her to know what is going on as part of that process of making amends.
Make it very clear that you are out of this, that you have no intention of going back into it, and that you are sending this and putting it in writing as a way of making that intention into the equivalent of a written guarantee to her.
Give yourself time to edit and cool down.
Then take this version to your bank, make a copy, have the copy notarized, and send the notarized copy. To her. By registered mail, with her signature required for delivery, so that he can't intercept it and prevent her receiving it. You can tell her you've notarized it and kept the original in case she ever needs to cite it for legal purposes. [This implying that she might use it as evidence against him. That being something you'd consent to. And you are protecting yourself by sending a notarized copy so that it cannot be falsified by her or him, to cause you damage later on.]
Just suggestions, but I'm serious about the registered mail thing. You want to make sure that he has no ability to disrupt this. Also, if he has the gall to show up at your doorstep again, you have documented your disinterest in no uncertain terms, and can use that if you need to.