I work at a dealer and saw a similar trans failure, similar mileage occur after its original trans fluid was flushed so I know that it can happen. I've also heard of that occuring elsewhere, always with higher mileage trannies that never had the ATF flushed before. Believe it or not, there's really nothing that your dealer did wrong nor could they prevent that trans failure. With higher mileage trannies that have never had the ATF flushed, it then can become a gamble to flush the ATF. If done at regular intervals starting at lower mileage, it's normally a good thing.
Your only alternative at this point is to have them (or someone) replace the trans. I'd personally steer away from used trannies due to the labor involved and that you're (again) taking a gamble with a used part.