You already know the solution, boundaries; you have to set them.
The friend is jockeying for superior position with you against your daughter, which, by the way, yes, it is weird.
25 year olds say f^c*. It's not earth shattering. Just tell her "that is my daughter, I will always be on her side, if this the hill you want our friendship to die on, so be it. I am sick of hearing about this. Period. There will never ever be a resolution that is satisfactory to you. Bring it up again, and we're done. If you cannot live with that then I understand, and we're done." She will either back down or double down. If she backs down you still have a friend. If not, there's your answer, walk away. It will not get better.
Your friendship is based upon her having no other friends, if I read that correctly. Anyone having no other friends is kind of a big flashing neon sign about something, usually social graces.
If anyone spoke poorly about, or to, my daughter, or to me or to anyone else, they would never be allowed back in my home and their number and email would be blocked. So why are you so hesitant? Does your daughter also have some kind of a disorder?
It reads like this woman was pestering your daughter until your daughter got testy, and that seems normal. Your friend, who I assume is the older one, so, theoretically, the more mature person in the exchange between her and your daughter, has taken zero accountability. Still, you won't fix the problem. Absent any other info, there is only one side, your daughter's.
Your hesitancy to fix this problem begs the question, is she actually YOUR only friend? Is this a potential loss you don't want to face? There is nothing wrong with that, but it would explain why you're so conflicted about protecting your daughter.
You don't have to parent to your friend's satisfaction, only to your own, and it's none of her effing business. You say to your daughter what you need to say to your daughter and only to her, if there's anything to say at all. Your daughter is an adult and she will decide what kind of a relationship she wants, if at all, with that woman.
I am personally against telling young women to stifle and much prefer teaching them how to rephrase so they don't lose their voices because somebody's panties are in a bunch over the word f--@, or they are perceived as unbusiness-like which is valid, or unladylike, which is not valid, but a way to silence women.
Good luck walking this tightrope! I'm rooting for you!!!