☆.A.
I think role playing with her might help.
She can say things like "we all have more than O. friend" or "you're my friend and Suzy is too. Do you know her? Susie, this is Mary" etc.
If this "friend hoarding" is extreme (sounds like it is) I think the teacher needs to be MORE aware of it. I know a little girl that was having the same issue with a girl in her class and it got to the point where the parents AND teacher had to get involved.
I tell my kid that he will meet new kids every year and some will be friends, some will be buddies, some-merely classmates. I've always told him that he doesn't have to like everyone but every kid has an equal right to be there, so be respectful.