What do you do when you watch your best friend of over 20 years making the biggest mistake of her entire life and she is destroying her entire future over it?
One of the hardest facts to accept in life is that people can only save themselves. You can't do it for them.
In my salad days I had a Jesus complex a mile wide and was determined to save my first wife from herself / her demons / whatever. This was a huge factor in keeping me in a dysfunctional marriage for 15 years, which was about 14.5 years and two children more than necessary, even ignoring that I should never have gotten involved with her in the first place.
I was warned, loud and clear, by multiple people. I did not listen. I was unwilling to believe them.
Like me, your friend may think she can transform this toad into a prince, or she may think she's only worthy of a toad. Either way, she's going to have to figure it out for herself.
Like me, you can't make your friend hear you. In fact, the more you try, the more you'll probably drive her away.
She will know who has her back, when she eventually comes to her senses.
Now you know how I feel when I see women that support Trump.
I read your other post first so I understand what this is about.
We can't control other people, only ourselves. Losing a long time friend under these circumstances is very disappointing, but it sounds like your fight to help her is mostly hurting you now.
Whatever she needs to learn, she'll do so in time.