I have a good friend who I have been close with for many years. She considers me her best friend, or sometimes diary. She has never said this, but in many ways that is what I am.

This friend is beautiful, smart, successful, charming, funny, etc. I like her. Ok.
As for her flaws, she is morally or ethically out-of-sync with societal norms and myself. She justifies every morally-out-of-sync decision and falls forward, smashing the next ethical boundary. She is aware of this, by the way, and doesn't care. She has told me as much. For example, she has cheated on every beau, has an affair with a married man, and is just very selfish in general.

I have the good fortune of living far from home and only visiting home for holidays and important events. When I go home, she and I get together. I currently have a wonderful boyfriend who she has never met, but the next time I return home, he will be coming with me.

I do not want to introduce her to my boyfriend at all. I know her. She is competitive and will get a rise out of flirting with him and trying to make him like her. I trust him, so the problem isn't fear that she will "steal my boyfriend." Not that she wouldn't have an affair with my own S.O. if given the chance. She "loves me" but I am not exempt from being a casualty of her selfishness. But back on point, I want to avoid this entire scenario because it will cause friction. I know me, too. I will bristle at her behavior. She will sense the friction and force a conversation to resolve the issue because she "loves me," but the conversation will just be her minimizing her actions and painting herself as a paragon of perfection and me as perhaps overly sensitive (and maybe paranoid, or possessive, or whatever is her flavor of the week).

If I don't introduce them, she will ask me directly about this evasion. She will ask me if I am worried he will like her (she thinks every man who meets her is in love with her) and try to reassure and manipulate until she gets her way. Like the time I kept trying to avoid being introduced to the married man she is having an affair with. She sensed the motive for my evasion, confronted, but had little leverage to force me to be an accessory to this affair. But in this new situation... where is my leverage to avoid this introduction?

How the hell do I get out of this? When did I become this weak?