Borderline Personality Disorder.

http://www.bpdcentral.com
http://www.borderlineresearch.org

Yes, the best thing you can do for yourself is end the friendship. Prepare
to become "the enemy" and to be talked badly about to your other friends,
perhaps even harassed or stalked, but eventually she will lock on to
another target (someone she loves until she decides she hates them), and
you'll be free.

[[This woman I've known for almost 10 years is absolutely a nutcase.  One
minute she likes you, the next minute she doesn't.  Sometimes she's all
"hell yes!" other times she agrees with you then turns around and bashes
you
to other people both publicly and privately.  I need to turn off this
"relationship" and get her out of my life.  There is no excuse for her
behavior.  Thank you for letting me get this off my chest. ]]

There are some people who it's best to limit interaction with them to
"Hello", "goodbye", and in between that talk about the weather and
everyone's health. This lady sounds like she fits that description.

If she's acting this way because of some sort of mental illness, she might
not even be able to HELP herself when she acts this way. But that's no
consolation if she's turned around and said something rotten to you or
about you. :o(

Purrs that you can relegate her to a place in your life where she'll cause
the least possible trouble for you.

Donna, Captain, and Stanley

Yes, Been There is right.  Classic BPD.
You won't be the only one to notice this (unless you and your friends are only
19).  The best thing you can do for her is tell her to get lost and tell her
why.   DO NOT accept an apology.  Get his kook out of your life.
These are the people that troll support groups.
Any hassle will be short lived.