It's not your fault, nor should it be any of your concern even if it was.
She chose to do what she did; if that was unacceptable to him then she shouldn't have done it, and she obviously knew it was because she hid it from him.
If we
were going to say it was your fault, you should be proud that you've saved someone from a painful trainwreck of a relationship. She is going to hurt whoever she is with, whether that's you or him or someone else.
However, it's not. If he dumped her for her choices, he knew he had to GTFO to protect himself. It's her fault.
Most of all, though, none of it is your concern. You owe nothing to her or to them both. Your only reason to give a fuck is the history and lingering emotions, this is not part of your life or part of the life of someone you should care for in that way.
In the future, avoid meddling like that -- not because it's your responsibility to anyone else, but because it makes you feel bad.