My boyfriends' and my ex wants to get back with us, behind her new partner's back

My boyfriend of 4 years and myself (F) tried poly with a girlfriend for a few months. She later found a man she decided to be monogamous with, but I recently found out she still wants to be part of our relationship but not telling her fiance. I don’t know what to do, because to me that's cheating and dishonest and it makes me uncomfortable, and she didn’t let us know about the engagement until after we slept together several times. My boyfriend says it's her prerogative but idk.

It’s not that this is cheating and dishonest “to you,” it’s cheating and dishonest period. This is not an “agree to disagree” situation! Since she is an independent agent of her own humanity, it is indeed her prerogative whether she wants to cheat on her fiance, but it’s your prerogative whether you want to be part of that! If a situation feels unethical and makes you uncomfortable, do not participate. It was a violation that she had sex with you under conditions (her presumed monogamy) that she knew you would not consent under. She is not treating you with respect. Do not be involved with this person.