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When we were together with my ex I have suspected that she idealized her best friend. But in a more personal way...

like sometimes I thought she would replace me for her.

Is it possible for a pwBPD female to find the emotional support and loving feelings from her best female friend?

In other words to be attached to a friend?  But without the physical thing e.g., sex or being lesbians.

In particular, the best friend is older than her, possibly hermit and maybe sees her as her bf.

As if they both have replaced their need/desire for having a bf by being together as friends and support each other emotionally.

Does this sound any reasonable?

Yes, what you're describing is very similar to me. My ex would completely idealize some friend of hers, say she 'loved her new best friend,' and constantly make new posts with this person. She had known this person for 1 month - 3 months and they were BEST FRIENDS FOR LIFE nothing can ever break us apart lunch everyday friend, and then something would happen and that friendship would crash and burn... .and then she'd move onto the next one and repeat the cycle. My exBPDgf was exactly in your position, but you see she lacked a true sense of self, she needed somebody to latch onto establish a sense of self, but it wasn't rooted in herself, but in whomever she was idealizing at the time. I didn't realize it at the time, but she was latching onto a new bf and was becoming bored with me because I worked all day, went to grad school at night, and was generally pretty much a realist about life and establishing a secure financial future but working hard to get there. She wanted EVERYTHING and I mean EVERYTHING I wanted AT FIRST when things were great in the beginning, but I guess things had run their course because she then idealized a new friend, who said you can have this and that and party and why would you want that, and since she never had a true sense of self, then the person she idealizes at the time is in the right for her, and thus I was discarded for something new as well as new boy toys/victims, but it started with her best friends' convincing. Really, my exBPDgf was never capable of making a decision on her own, and really either only went for what was easy, or went with what she thought was best because it would please again whomever she was idealizing atm.

I suppose this was a fact for my ex. I could say that with any interaction with a "friend" discussing an issue, she would agree with her friend's opinion.

No matter if that was wrong or what we had said before about the same issue, had agreed and had found a consensus. After coming to me she would try to make me agree with that opinion.

I don't understand why she would "sacrifice" me with her best friend which was also female. I could provide her with more things like sex etc... .

Attachments are attachments to them. They don't necessarily draw the clear lines between types of relationships that emotionally healthy people would (remember the lack of boundaries). Just think about how many people here have had an ex say horrid things to them and the. Turn around and say "I want you to be my best friend." Idealization is a tool for forming an attachment, fusing with another self.

So, whether or not you're reading this situation exactly right,  it's reasonable.