There’s an old joke where a lawyer tries to argue with a judge. “Your honor,” says the lawyer, “my client was out of town on the day of the murder. And if he wasn’t, he was not at the scene of the crime. And if he was, he didn’t pull the trigger. And if he did, he’s insane.” Your friend also sounds like she’s trying to have things both ways: there’s nothing wrong with her actions, she says, and she’s not responsible for what she’s done even if it was wrong. Your friend may not be guilty of adultery (depending on how we wish to define that term), but she’s certainly guilty of aiding and abetting.
As people living in the 21st century, it is perhaps incumbent upon us to recognize that monogamy is not for everyone. It is not incumbent upon us, however, to condone cheating. There’s nothing wrong with having multiple partners so long as everything is consensual. But if one person has explicit reason to believe that they are in a monogamous relationship while the other has sex with other partners in secret, then the latter is exposing the former to risks to which s/he did not consent. Non-monogamy is one thing; cheating is another.
Do people do crazy things for love? Absolutely. But that is an explanation for her actions, not an excuse for them. She deserves your sympathy, but not your approval. It would be one thing if her “crazy actions” were merely out of the ordinary or extreme. The problem, though, is that they are not merely those things. Her actions are dishonest and inappropriate. If I were you, then, I would not give up my position in the name of friendship. A friend is someone who tells you the difficult things that you need to hear. I would be clear that I cannot make the decision for her, but that I had strong opinions regarding what the correct and incorrect actions were in this situation.