I have a few questions to ask a very good solution if the situation matches.
Was your best friend being very out of line when she turned on you? Were you being out of line when you told her to stop complaining, or were you justified? If this is true, then your Best friend could just be throwing a tantrum in order to gain more control in the relationship. This has happened to me before.
One of my chick friends was having one of her usual mood swings (Waa waa, my life is so stressful and pointless), and I was just ignoring her, deciding it was best to not get involved. The teacher in the class (who my friend hates) noticed she wasn't doing any work and was in a mood, so asked her what was wrong. I interjected saying "She's just annoyed because she can't intergrate these equations."
Suddenly, my friend said "Thanks CaptainJ" in a really pissed off tone and stormed out of the classroom in a strop. She spent the next 2 days ignoring me, even saying in my presence but not directly at me that she was angry at me. I couldn't give a ****. I realised she was just throwing a tantrum in order to gain more control in our relationship, and make me her consoling b1tch. So I was indifferent, even made fun of her tantrum, because I knew what she was trying to do and knew she was unreasonable.
She then IMed me, saying we needed to talk and began a premade speech about how "inconsiderate, pretentious, arrogant" I had been. I then, suddenly to her surprise, went into full attack mode. I called her out on her *****y little tantrum and said I was pissed off at how she tried to blame her mood on me. She was totally taken aback by this sudden change (I used to be her consoling friend) and she began crying and apologising for how she acted. I left saying I had better things to do and she texted me asking for my forgiveness and all this crap. To be honest though, I wasn't actually angry when I said those things to her, I was actually laughing, at how I had turned the tables on her, and gained power in the relationship. I thought she would hate me forever for attacking her, but it had the adverse effect, and she was the one crawling for my forgiveness. The trick to my argument with her was that I didn't try to justify anything logically, I argued on an emotional level, because Girls will not accept a logical argument in the heat of passion. I didn't need to justify anything giving a logical reason, I just called out all her bull**** and attacked her. She threw me a trick question "You don't even know how I feel". NEVER TRY AND TELL A GIRL HOW SHE FEELS, YOU WILL ALWAYS BE WRONG, and she will win. I sidestepped this by simply saying "I don't need to know."
So, if you feel this girl is being unreasonable, you must maintain your integrity. She has probably given an extremely *****y account of you to the girl you were looking to date. If I were you, I would act indiferent to her tantrum, ignoring it, and when she tries to talk to you about how you made her feel (She will), you attack her and call her out on her bull****, call her manipulative and *****y for talking behind your back and influence her friend etc. Girls give a strong initial fight which intimidates many boys who fear losing the relationship for good with the girl. But if you go on the offensive, you will find the girl suddenly loses all confidence and will soon be apologising to you. You must not admit you were ever in the wrong, you were right and justified in your actions. This will make you look good to both your friend and her friend, because you stand by your principles and have strength to not be manipulated by btichy tantrums.