Your friend clearly likes Sarah even if you don’t. Sometimes people have their reasons. Your options are to see your friend without Sarah present. If that’s option isn’t available (although I don’t get it, are they romantic couple? If not are they ever apart?) then perhaps you might give up on that friendship. I still kind of don’t understand this dynamic. Does sound a bit high school.

Is it common for you to have such dynamic with friends? Or it’s just this girl?