[TonyB06]

It's always better to know.

But it's not clear from the narrative whether they were an "exclusive" couple. Exclusivity, something central to a lasting relationship, shouldn't be left to assumption. Yeah, dude's actions look a bit sheisty here, but if they hadn't agreed they only wanted each other, then either is free to at least look around. His actions suggest that he wasn't as sure as the woman (your friend) was. That, by the way, is nobody's fault, it just is.

As to the internet "cheating" thing, I agree (my opinion's changed a bit in this area) with ladylike that "a mental connection/affair can be more damaging to a relationship than a physical/connection affair." As we mature we tend to look deeper in others, making it possible to connect on deeper levels -- but if you're connecting with someone other than your spouse, then "Houston, we have a problem!"