That is not what bar raising is intended to be. There's always a bar raiser in an Amazon interview loop who is not affiliated with the hiring team. They ask the same kinds of questions as the rest of the interviewers in the same way, but they're not influenced by how much the team needs to make the hire, which in theory prevents the team from lowering the bar just to get a warm body in.
I'm not saying your friend didn't get a bad interviewer. It's just an interesting demonstration of how risky it is to generalize from anecdotes.
It was a generally accepted practice based on talking to the folks who interviewed with Amazon during my wife's time at Kellogg that one interview would be conducted by someone (an interviewer who had done 100+ interviews) who would be unpleasant in some way.
I'm guessing the way Amazon interview for business roles is somehow different than how they interview for specific team positions. As I understand it, MBA hires aren't filling specific gaps on teams, since the hiring process is so long and involves an internship. There wouldn't be a "team" to lower the bar for, in other words.
I'm not saying your friend didn't get a bad interviewer. It's just an interesting demonstration of how risky it is to generalize from anecdotes.