I recently had the serpentine belt change after about 85,000 miles. What happened next was that after only 3,000 miles the ideal pulley went bad and destroyed the belt and left us stuck on the road. Was this something that was caused by the mechanic who changed the belt the first time or was it just time for the pulley to go???
If it was an “ideal” pulley, it never would have failed. (To me, ideal pulleys ideally last forever.) I suspect instead that it was an idler pulley. It is not unheard of for those to fail. How many miles on the car? I really doubt that it was caused by the belt change. It could be that the mechanic changing the belt should have noticed the worn bearings in the pulley and suggested replacing it too. Perhaps you could get some discount on the new belt and pulley because of this?
Some mechanics routinely replace idler pulleys along with the belt, just for this reason. I doubt there’s any connection between the belt replacement and the pulley failure. It was just time for it to go.