That's is really, all in the title.
There's four friends that have been close for a number of years- my friend we will call Angie had an awful marriage, newly divorced and has been going out and dating and rebuilding her and her children lives- we have all supported her- looking after children, hand holding when the divorce proceedings were stressful etc.
I went for a catch up with our friends minus Angie. My friends were telling me at a birthday party held at her(the friend who was at the catch up) house which I was not able to attend Angie got completely drunk and slept with the party host's husbands friend.(Who is married with two young DC)
They all found this hilarious- furthermore (the 'couple' went into the lounge)some of the guys thought it would be a scream to go outside and film what was happening not realising Angie and the man were having full sex- after they passed it around it was deleted immediately on the sayso of my friend. This conversation continued where they all laughed at said "ah bless Angie- she's had a terrible time she really needed that- he's quite a catch!" etc 
I was shocked. I didn't think that my friends were like this. It doesn't help that my partner cheated on me years ago when my DD was baby, I know I am projecting massively. I cannot help myself I just feel really sad for the partner of the man who shagged Angie- she is not my friend but our paths, included that of her DC, cross very frequently.
Since- Ive been told via group text- not in person, Angie has gone to conferences and slept with one married man- she did this knowingly, she knows how much this can destroy a family. I am upset at her but equally my friends who support this and find it salacious and hilarious.