You did the right thing. It would NOT be your place to tell her boyfriend because you're not the one that cheated on him. He is.
It sounds like it was only a one-time thing, right? So by telling him it would only unecessarily hurt him. The only time she should tell him is if she's planning on leaving her boyfriend for this guy. But if it was a one night stand (something she'll never EVER do again) why do that to her boyfriend?
And don't judge her. Even if you REALLY disagree with what she did.
It sounds like it was only a one-time thing, right? So by telling him it would only unecessarily hurt him. The only time she should tell him is if she's planning on leaving her boyfriend for this guy. But if it was a one night stand (something she'll never EVER do again) why do that to her boyfriend?
And don't judge her. Even if you REALLY disagree with what she did.