I've been watching this thread and here's my perspective for what its worth. You can't change Sarah's personality. You can't make them stop being friends with each other. You can voice your concerns but it is unlikely your friend will unfriend Sarah because you don't like her. I wouldn't do that.

My advice is short and simple: let it go. Maybe its the end of the friendship. Maybe its a break until something changes. If its making you unhappy, let it go.