1992 Buick Park Avenue water pump

While driving my car died. Under the hood: serpentine belt had come off (not damaged) and coolant had leaked out. I thought maybe a hose had come lose spraying the antifreeze around the engine causing the belt to come off. My mechanic says the water pump froze up. She proceded to show me a shredded belt until I told her mine looked fine and was in the trunk of the car. Should I trust her?