half shaft torn boot

The CV boot is like the skin over your wrist, the CV joint is like your wrist. If the CV boot is split the joint will wear. The CV joint at each side of the axle allows the axle to transmit the drive from the engine through the gear box reach the driven road wheel, The Cv joints second function is to allow flexibility during suspension rise and fall, and thirdly it allows for flexibility during steering operation. Ask them to clean the joint or inspect for wear prior to replacing the axle if it is not making noise but if the joint is full of debris rebuilt axles may be the best alternative.