Nowadays most people cheats sooner or later (and this is well known), and you take it as a "shocking discovery"?!?
I mean, finding out your husband is really an alien, or he’s married to another man, could be considered "shocking". Cheating - disturbing as it might be - it's something most couples have to face eventually.
It's like one driving, having a car crash, and be totally flabbergasted that it could have happened to her... :-o
I don’t want to sound callous or disparaging, but… c’mon! An adult should know how life works, and that no spouse is perfect. Maybe your friend should have realized she wasn't living in Disneyland or in a fairy-tale.
BTW, to the ones thinking “Cheating it’s the exception, not the rule!”, I’m afraid you’re wrong: most people do it, sooner or later. If you Google “how many people cheat”, you’ll find many studies that support this (the first result reports at least 71% of men and 82% of women admitting cheating: https://techreport.com/statistics/lifestyle/cheating-statistics/ ).