My boyfriend in high school cheated on me. Bad. There was a video played at Saturday morning football practice. The girl was our chemistry teacher's daughter. The principal got a hold of it. My BF and the girl were eventually charged with child pornography kind of bad. But I was dumb and didn't believe what people (including teachers who saw it) told me. Eventually I figured it out. But my best friend (still to this day) who was a mutual friend of us both, decided he wouldn't tell me bc he didn't think I would believe him. I still get angry with him at times for it (rational or not) and really truly wish he would have said something. Since then I never hold back when someone cheats. Even if I barely know you. If you want to suck as an SO, it's not my problem. As someone who's been there, I want the innocent party to at least have the facts to make their own decisions with. Nothing worse than looking like a fool and finding out later your friend knew and never told you.
I'm not saying it should be that easy of a decision for you, just throwing my two cents out there. Maybe try and convince her that it's best he finds out from her and that he will be much more willing to forgive her that way (not saying he should).
I personally would be disappointed in my friend for doing it. Which would make me feel that the guy is much more deserving to hear the info, than my friend is to cry on my shoulder about something I completely disagree with.