I should have been more specific. You have known each other for 20 years, how many years of those 20 years were you in a romantic relationship then?
You were out of status, so a marriage with a USC was a way to fix that and in that short period of time you already had an affair. You only just received your greencard and now want a divorce. From an outsider perspective these factors are already a little sketchy, and she might have even more info, so again: she might have good grounds to want to report you. Even if she does, it might not make a difference for your I-751 with a divorce waiver though.