I would just encourage you to prepare yourself emotionally for the possibility that this guy wasn't the first, and that her infidelity goes back further. Whether you snoop further (I agree, you probably shouldn't) or whether she confesses it at some point, I just don't want to see you take another punch to the gut if that turns out to be the case.
I'd also suggest that although you're thinking you're "not even going to get into it with her" right now, she IS going to ask you "what's up?" and "why are you acting this way??" You've been a best friend to her for so long now, she's going to notice the emotional pullback, and she WILL ask you about it.
