If there was one thing I wish I had realized to be as axiomatic as the sun rising in the East and setting in the West prior to BD day is that when a WAS informs an unsuspecting spouse of their intent to file for divorce and, "oh by the way there's someone else and I wasn't really out running errands today like I told you I was going to do but I was actually at OM's place", they are SO FAR GONE already, SO Emotionally Checked Out and divorced from that spouse and probably have been for quite a while, that the only logical response for the poor LBS at that precise moment is to probably say "well, sorry it's come to this, I wish you every happiness, it's been a great so many number of years, give my best to OM, but I'm outta here, like NOW!" Then disappear, make yourself conspicuously scarce and visit your lawyer.
Also, bravo for use of the word "axiomatic".
H 34 W 29 BD 3/12/18 Divorce Busted Spring 19
It is not things that bother us, but the stories we tell ourselves about things.