Why does my girlfriend NOT see how toxic her best friend is?

Anonymous
My girlfriend's best friend (let's call her P) is very toxic and she talks to her only when she needs something. P recently met and had drinks with my girlfriend's ex inspite of my girlfriend trying to talk her out of it. My girlfriend's ex cheated on her and it was a painful betrayal. But her own best friend is now betraying her by going to meet her ex (for the first time. They are not friends or anything. P just wanted to be friends with him to spite my girlfriend which she confessed) which is messed up. But my girlfriend can't seem to realise how toxic P is. She gets upset, feels low and depressed because of P and it's starting to come between our relationship! But I can't force her to stop talking to her. That would be wrong. I just want her to realise that P is only taking advantage of her and she doesn't care how my girlfriend feels.
Why does my girlfriend NOT see how toxic her best friend is?
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