Is she my friend?

I have a "friend" just like that. The best thing 你 can do is avoid them and do your best not to irritate them in any way. If she starts being a bag to 你 或者 your other friend, tell her off and break off the friendship. I had to do that the other day. My good friend did the telling off and, let me tell you, there were 更多 than a few choice words flying around..

Actually i disagree with above, any friendship is worth the pain and strife as long as it's kept under control.
你 need to sit your on and off friend down and get some ground rules in force. Explain to her, that 你 value her opinion and will listen, but under no circumstances will 你 tolerate her bad mouthing your other friend 或者 being mean.
Try to hang out with both of them equally, but if they don't get along try to avoid them meeting. And don't lie about who you're hanging out with. If your on and off friend calls and asks what you're up to, be honest and say you're with your other friend (when and if 你 are) and she must accept that she is not your only friend, so she shouldn't be jealous.

If your on and off again friend can't accept these rules and ignores you, then i think it's time to leave her behind. But i'd sort it (or try to) first. All friendships are difficult to handle and understand and most of them are 更多 bother then they are worth, but if 你 trust this friend, and value her friendship, 你 will do right 由 her, even if she won't 由 you.

I had a friend just like that. What I did was I told her that she was making me upset when she was being mean and I told her that if she didn't change I'd not be her friend. The problem was, was that we spent too much time together so now we don't spent as much time together and were great 老友记 again.
I'd talk to your friend and tell her that 你 feel sad when shes mean. If she's a true friend she will sit and listen to 你 and try to change for you. My friend didn't like some of the people I hang out with but she was okay with it after we talked.
I wouldnt ditch her straight away, try and talk with her because all friendships are worth it in the end.