I have this problem as well…my best friend has a really good friend who is a total bitch. We’ve hated each other since childhood…why, I don’t really know. I think it’s because she’s promiscuous, bitchy, and isn’t that smart.
I talked to my friend about it. I told her that I hate Margaret and she hates me and nothing is going to change that, and I have no desire to change that. I told her that I don’t want to put her “in the middle” because I care about her…so I would just like it if we could hang out sometimes one-on-one without Margaret…and she could do stuff with Margaret when I wasn’t around.
It worked out really well. At birthday parties, we just avoided each other and never spoke a word to the other since. It’s still that way now.