OK I think I have a good handle on this. I just need to listen, listen, listen, thank her for telling me, and then give her a very brief rundown of where I stand.
I don't even necessarily think you have to do that, MrM (and by the way, it's time you changed your username to something more positive -- you're not making so many mistakes anymore!). Don't feel obligated or compelled to reply, other than with the "thank you for telling me" stuff and maybe a "you've given me something to think about" or some such.
Or maybe "My position hasn't changed, but I do thank you for being honest with me, and giving me something to think about."
THIS IS IMPORTANT. She is going to be looking for some kind of response from YOU, the purpose of which is primarily to ease her pain and relieve her guilt... to make her feel "he's okay with what I'm doing."
It is not your job nor your desire to rescue her from the consequences of her infidelity.