C.B.
I have a similar situation with my 9 yr old daughter- Every time she invites this friend over- something "comes up" and she can't do it. She even was supposed to have this girl sleep over at our house last year, and on the day of the sleep over, she didn't walk home with my daughter. My dd said it was because the friend had forgotten her note from her mom. So I called and left a message for the mom, letting her know that her daughter was not at our house but the school had sent her on to her day care as usual because she didn't have a note but that we would be willing to come pick her up later.
She called back about an hour later to let me know that her daughter had made plans with other friends at the last minute, so she wasn't going to sleep over. This was the final straw for me. I just told my daughter that her friend was unable to come and then Daddy took her out to a movie. She is still good friends at school with this girl, but I have really encouraged my dd to make new friends and invite them over. We have also put her in girl scouts to get her to meet new girls- that are nice! My dd still asks if she can invite this girl over, but I won't let her- I just tell her we have other plans or ask her to invite someone else over.
Good luck- I would keep encouraging your dd to make new friends and invite them over and just stop inviting the other one.
~C.