Yeah, letting her get away with her cheating seems to be far to cheap and easy for her, but then again if I would be where he is standing, having "a very special one of a kind job", that needs time off, and a way to become rich, I'd also try to end this as quickly as possible. If our relationship continues in any form, there is a chance, that she sues me for money! It doesn't matter, if she has a chance of winning this or not, she would be troubling my efforts and that's enough to blackmail me into paying her out.
So, yeah. I would act be a bit more shocked, but tell her, that I expected that much and maybe that I would have been very close to send a P.I. investigating her. I'm still a bit "shocked", because the truth still hits me. I'd tell her I want a clean cut, but my share of the cake. Then I'd listen to what she wants, tell her what I want (which would be the very same things, she is willing to let go, except the things I feel she tries to get the better of me. I would want to take those things at least in the beginning, not because I need or want them, but because I know she would start to ask herself, "Why was that so easy?", and might to look into me. Remember, she cheated on us, so there is no reason not to believe, that she is now out there to get us. Then I would "give in" very quickly, because "I'm somewhere deep inside me emotionally distraught", which I would "try" to hide, and accept her counter offer, taking just a bit more she was initially willing to give up, except her initially offer was already very fair. Then I would have excepted it the way MC did. This way she believes she got away. and doesn't bother to further look into me.
She must have no reason to further look into me. Even better if she has reason not to touch the matter (me) ever again, e.g. because of fear I might want more from her. And then I wish her happiness. As long as she is happy, I am, because she stays away from me.