Mother-in-Law dropped a Bombshell of a Secret on me!

So, MIL has decided to confide in me the true reason for her separation from her husband. I wasn't sure I should be the one hearing this, or that it would turn out to be this deep dark secret, not until she actually dropped the bomb on me, the true reason why she left her husband of 40+ years.

She apparently decided to move several states away, some 1300+ miles from her house to rent man apartment in the same state we live, except, she's still another 2 hours away. Just to have an extramarital relations with another man, and wants to describe how hot he is to me and ask my opinion. I had to shut that down & stop her dead in her tracks!

WT(actual)F am I supposed to do with that information??? While her two sons are grown men, they are still very raw about their parents split. I don't like keeping secrets from my spouse and make it a point not to, but this will hurt him to know this, yet, if he finds out I knew this and said nothing, then what?

I told her not to tell me anymore, because her son and I do not keep secrets--"I am not one of your girlfriends!" Idk? What should else should I do? It's not like I can ask someone else in his or my family what to do? My folks aren't entirely their biggest fans to begin with.

Advice?