Most Helpful Girl
Her justification was most likely an attempt to soften the blow of being caught. There is no justifiable reason to explain cheating. She claims you don't understand, but it appears she's the one who doesn't understand. You can't be pressured to do something you shouldn't or don't want to do.
Most likely she saw an opportunity she didn't want to pass up. Perhaps he appealed to her physically, or just had a magnetic personality. Regardless of the reason, for that moment, she elected to forget she was in a committed relationship and made a conscious decision to act on his advances.
Relationship commitment is important, with trust being a key component. Guys will come and guys will go. While women may get more opportunities, its rare that we can find a decent guy that will be with us for long term when so many of them just want sex. She took a major risk in destroying her relationship for a few moments of an immoral guilty indulgence.
Granted a guy may look better, have a better personality in terms of sweet talking women, but that doesn't mean he is actually better in character nor that he has long term interest in her. Taking a risk like that will kill trust, cause undue pain and kill a relationship. IMO you're better off without her.