I'm not sure of the usual meaning given the "east side" slang term "prat." My read on your response to the situation you were presented with is that you acted reasonably. If you had concerns about keeping the trio together, it would have been wise to explain the circumstances to the whistle player. She might have backed out of the gig graciously, had she been given the chance. It does sound as though the cohesiveness of the group was already under stress, so maybe you should be courteous and move on. Band break-ups are a lot like divorces in the emotional sense: rarely, if ever, easy, but sometimes very necessary for the good of all concerned, though it may not seem so at the time. One way around such problems in the future is a simplified "contract" or agreement that spells out what happens when those unexpected situations arise, and who decides what to do.