I would just say... we do not really have any context on the relationship here.
Is this a one-month old open-ish relationship and you're just like "whatever, I didn't view it that seriously myself yet either"?
Is it a three-year-old fully monogamous relationship where you went through a rough-patch, then reconciled, and she took in some fresh cock in the interim?
Is it a five-year marriage where things have never really gotten all that rocky but one steamy night at the office she took Coworker John into her cubicle and made illicit after-hours love?
It is pretty hard to judge how large a breach of trust this was or not without context on the relationship. You seem pretty chill about it, so maybe not that big of one?
If it was a serious, long-term monogamous relationship, with no breaks or anything like that, and you were both being faithful, then it's a big breach of trust, and you look bad / lose a bunch of respect letting it pass.
If it's casual, or on-again-off-again, or something else, that's a different story.
Chase