They say B introduced A to C and got left alone.
AB + BC = AC
They sympathise with B.
But maybe there was a reason why A, B's best friend, and C, B's partner, left B and became besties together?
Maybe B changed? Maybe B wasn't the person they could any longer trust in?
Maybe they realised that B isn't someone they could survive with?
Maybe leaving B was the best thing....
But remember; B was the reason A met C and became besties....