yeah. I mean it's unfortunate but there's nothing you can really do at this point tbh other than advising her.
you aren't going to change her mind, she has to decide what she wants for herself.
often 'we accept the love we think we deserve'. so yeah.
first of all, if they were willing to ******** another person when they were with you then they don't love you the way they claim to.
they clearly want/desire/love the other person more if they were willing to betray you like that (assuming we're talking about a monogamous relationship).
then there's the fact that if they were able to do it the first time, despite what they say/think, they're capable of doing it again... and again.
typically people are repeat offenders. just sayin'.
why do you ask anyways?