Pull out All the Stops

Max is busy preparing a surprise for a friend's birthday, involving decorating and baking. The conversation shifts to explaining the idiom 'pulling out all the stops', which originates from pipe organs where stops control air flow to make music louder. Max then asks for help and reveals more about the friend, ending with a birthday wish.

What does Max mean when he says 'both the musician and the instrument are trying as hard as they can'?

Why does the other person in the conversation ask about Max's best friend?