Alternator type

My car broke down and the recovery service man advised that the belt snapped and there was no fluid in power steering tank. The car was taken to a garage and was informed that a wrong type of alternator was fitted in some time ago and it needed to be replaced. The alternator appeared to be a fairly new one, but I do not remember it was changed since I bought a car. I would like to know if the diagnostic is accurate because the recovery service man told me I just need a new belt and fluid. Is it possible for an experienced mechanic to fit a wrong type of alternator. Thank you