I hope for Rinne that Kanon never finds out what she said to Saki, Seeing how Kanon responded to Saki's offer it should be very clear that Kanon wouldn't be all too pleased.
I honestly expect little sister to greet them with an "I thought I told you to never visit her again!" the moment they arrive, and Kanon putting her in her place. Especially seeing how quickly this manga likes to solve such things.
We already know that the scenario Rinne was describing has happened before —Kannon's previous best friend really was just using her as a status symbol and dropped her hard once it became too troublesome— and she was right about Saki hiding something, she just came to the wrong conclusion about what that something is.
I don't remember seeing any indication of the friend using her (but feel free to point me to the relevant pages if I'm wrong), but it certainly got too hard for her eventually.