Sorry, although I'm a regular Mudcatter, I had to toss my cookie for this one, in case the person in question reads this:

A band member who regularly tries to out-argue her fellow band members on every matter of taste and judgment that comes up.

Apparently because she is the only one in the band with a music degree and knows lots of music theory and history, she thinks her opinion should carry more weight than anyone else's. She always knows, or thinks she knows, what chord goes where, whether a certain grace note should be played or not, and which of two versions of a tune is more authentic. Nothing is ever just a matter of opinion to her. Every opinion is either correct or incorrect, and hers is the correct one, and she is more than happy to educate you by explaining why.