I had my Alternator Replaced in June, then again March 20th due to an initial defective alternator. I drove 1300 miles since this was replaced and today the car starts shutting off and the power steering goes out. I was able to get the car safely off the road, but I heard a thump as I turned initially. When I got out of the car I looked underneath and the idler pulley had broken off. Was this a possible result of the initial alternator replacement, seeing as they have to get to that by going removing the idler pulley? Would they not have known there was an issue if so a month ago? And if they had, shouldn't the dealership have told me. I have a Cadillac SRX so you can imagine the cost to replace things because I go to the dealer. They replaced the alternator in March free of charge because it was within a year.
Obviously the parts are related, seeing as how the serpentine drive belt is associated with both the idler pulley and the alternator, but realistically the idler is just worn out and one has nothing to do with the other. The idler is a stationary pulley. It's only lot in life is to provide a pivot point and change the direction of the belt as it travels its given route. Unlike the tensioner pulley, it doesn't even get touched, except by the belt when the belt is being taken on and off during fitment. There would be no way anyone could exclusively rule out something happened during alternator replacement which caused it to fail, but if I were to put a percentage on it, I'd give you a 99%+ assurance one had nothing to do with the other. Things can happen, but realistically there is no "there" here.
If it's any consolation, an idler pulley is relatively cheap to purchase and to replace. If you have any mechanical skills at all, you could probably do this yourself. Not having looked at the SRX engine and drive belt system, most GM vehicles which have an idler pulley, the idler pulley is just one bolt (probably a 13mm??), then re-routing the serpentine belt, which will have the routing on a sticker under the hood. Probably a 10 minute job. You'd need some way to retract the tensioner pulley, but really, the job should be quite easy. I have no clue what the stealership would charge, but you're right in your assessment they will scalp you (in most cases). If they'd caught it when replacing the alternator, the cost would have been minimized because most of the labor involved would have already been covered (pulling/replacing the serpentine belt).