Friend Problems!! I'm so confused?!?

My friend and I have been friends for a long time now, but there was something that came between us, and I know how cliche this sounds, but it was guy. She was infatuated with this guy, so of course she wanted me to friend approve him. We clicked and had a little laughs, but nothing more. He called me every other night to vent and stuff, and my friend thought we began secretly dating, but the thing was...she never told me this. So one night at around 1 AM I get a text with HER SISTER calling me a backstabbing b*tch, I know, very mature. And then she posted things on FB about me, when I tried talking to her about it, she said that I hurt her, and that she could never trust me again. I don't know what I did wrong, I told her about the things he told me, he definitely hung out with her more, I supported her through everything, I absolutely WAS NOT secretly dating him, and when I apologized over nothing just to save our friendship she said it would take time to fix everything. I'm so confused, and I just want to know what I did!