I don't want to take anything away from the pain your friend is experiencing. I do just want to quickly chime in on the "infidelity is everywhere" aspect of the original post and some of the comments.
First, you need to remember that you aren't friends with a random sample of adults. For the most part, people make friends because of shared characteristics and experiences-- spending time at work together (where you're meeting people who are drawn to the same type of work you do), going to school together (where in High School you are going to share a ton of socio-economic factors with many of your fellow students, and in college you're going to be around people of similar intelligence, drive, etc). When you meet friends in your neighborhood, they are going to be way, way more similar to you than someone you'd meet if you put every adult's name on a slip of paper, shuffled them, and then drew one at random.
So if you look around and see adultery amongst your friends and other people you know, be careful about over-generalizing. There are plenty of people that live in very different communities, with very different experiences, and very different values. The only reasonable way to make a statement about how common infidelity is, is to use statistics, not personal experiences. Unfortunately, the estimates for how common infidelity is vary wildly, and there does not appear to be an authoritative source that you can turn to. I'm inclined to believe the studies that say around 25% of men cheat at least once during their marriage, whilst around 15% of women do so. This seems to be a common finding. But there are serious sources that claim the percentages are double (and even triple!) those numbers.
(I'd be interested in knowing how many of those are unplanned, spur-of-the-moment one night stands that are never repeated. To many people, cheating is cheating, but I consider the calculated lying and deception that goes into an ongoing affair to utterly destroy trust, so there is (to me) no possibility of reconciliation. A one-time, unplanned encounter, where the only lie told is the lie of omission (if the partner doesn't admit it) is still quite serious to me, as it make me question aspects of the relationship like respect, commitment, and love; but it doesn't completely destroy trust (to me).)
Second, you need to remember that as a result of being betrayed, you are probably consciously (like joining this website) and unconsciously (like looking for yellow/red flags in relationships around you, without even thinking much about it, in a way you didn't do before you were betrayed) going to learn about more infidelity than you would have if you hadn't been betrayed. That doesn't imply that there was some high level of infidelity all along that you were blind to; it means that whatever the underlying percentage was, you're now picking up on those cases where it occurs. A really common example of this that I imagine almost all of us have experienced or at least heard someone mention is when you buy a new car: suddenly, you see that car everywhere. It seems like every fifth car on the road is the same model as the new one you just bought. That's not because the new car you bought is way more common than you would have guessed it was, if you had stopped to reason it out; it's because your eyes are recognizing them and your brain is noting them, instead of just sliding right over them.
The reason I think this is worth my (now lengthy) response is that you materially hurt your prospects for having a healthy, happy, committed relationship someday if you overestimate how prevalent infidelity is. Mark Twain said it best: "We should be careful to get out of an experience only the wisdom that is in it and stop there, lest we be like the cat that sits down on a hot stove lid. She will never sit down on a hot stove lid again, and that is well; but also she will never sit down on a cold one anymore."
(Note: I don't know if that quote was spoken, and someone other than Twain wrote it down; or if it was something he wrote himself. In either event, I have taken the liberty of adding punctuation marks that are not present in the several places I found it with a quick Google search. Such is my hubris.)
[This message edited by Sordid at 11:25 PM, Monday, June 20th]