Hi you all, I just would love some opinions on this : Do you think this person might be non monogamous but haven’t realized it yet?

This person has been in a relationship for 8 years with her partner, but cheated A LOT, caught feelings for multiple people, fell in love with one (me) and struggles getting over me during the past 4 years which had her come back so many times in btw to get serious with me but lied about her relationship status. A year ago, I found out she lied about being single for about a year, and when she got caught she chose to stay with her partner to save their relationship cause it's been so long. I also found out they got engaged right before she came back to me insisting as she was trying to have a serious relationship with me while being engaged. Her fiancée apparently knew we were talking but thought it was meaningless flirting, and also knew when she'd came to see me, but wouldn't say anything cause "she didn't want to deal with that". She hasn't stopped reaching out this past year claiming she can't get over me, still confused but eventually she made her choice. She knows she can't cheat again and she's trying hard but it's hard for her as she has to resist and force herself not to. It’s over between us as she says it’s impossible as long as she’s with her partner but she has a hard time letting me go and can’t seem to leave me alone. She can't help herself from reaching out and she did again anonymously over the holiday behind her fiancée's back and said things she shouldn't have that would be inappropriate/disrespectful to her fiancée if not emotional cheating.

I just feel like this will happen again, not necessarily with me. Almost feels inevitable with her history. To me, as long as she doesn't try that lifestyle, she'll hurt people but maybe I'm biased by my feelings and she's just someone toxic. The fact her fiancée also rather get hurt and forgive cheating instead of maybe letting her love who she loves and be happy. Idk. I offered poly to them, as she would not leave me alone but her fiancée absolutely refuses, even tho this keeps happening.

I realize I can't force anyone to anything. I'm just looking for opinions and would like to know what you think or if anyone had a similar situation.