<human>: Some background:
 My friend "Katherine" and I became friends this school year, and we've been friends for a few months, but we became really close in that short amount of time. The only complication is her being bipolar and having depression, which can make her act as a less desirable friend, but I still love her as a friend. 

 We used to share a mutual friend, "Matthew" who she pretty much hates now, but I still very much enjoy being friends with him, as I've been his friend for a bit longer. 

 He had been used to being her friend for years, and is very familiar with how she treats people ("She'll come around, just give it time. She always does this.) They are no longer friends, and he's okay with this. 

  Their not being friends never affected mine and "Katherine"'s friendship; the three of us could sit and I could talk to both of them without them having to talk to each other. 
 
The problem at hand: 
  Because of a recent interaction with "Matthew", "Katherine" complained that I wasn't doing anything about his (admittably horrible) behavior and accused me of picking sides. I told her that I couldn't do anything about "Matthew's" behavior because that's not my job, and she got more upset. 
 
Eventually, this led to her not wanting to be my friend because "Matthew" and I are pretty much against her in her mind. 

 She's been avoiding me for about a week and "Matthew" has given me advice to just wait it out because he thinks she'll do what she usually does, which is cut everyone off, only to end up with no one, and then try to get her friends back. 
  
 It's gotten to the point where I just miss her and her friendship, but I still want to be friends with "Matthew" as well. Should I let her go and forget about her, or try to get my friend back?  Any help at all will be greatly appreciated.

Provide a brief overview of the text.
<bot>: Friend got angry at me for "picking sides" with a former mutual friend who angered her, and now she won't talk to me.