This just happened to my Buff Orp too... I got freaked out reading a lot of stuff on cannibalism! I quarantined the victim overnight last night and then went and got pine tar, a Flock Block, and some sunflower seeds today. My 4 hens can't free range because we live in the hood, but I think they're bored out of their little skulls in the run. Even though they have a *tree* to run around!
Anyways, I pine-tarred up the BO's behind and stuck her back in with the rest - we'll see how she does. I think it's a Blue Andalusian that's being aggressive, and I've read that Mediterranean breeds can be like that. I'm going to try to give them more "stuff to do" in there. But if it keeps being a problem I'm going to remove the aggressor next time.
I did also check for mites around the vent area, which I'd read was recommended. But my poor baby looked just like yours - it was right by the tail. :-(
Good luck - let me/us know if you find something that works.