Why would you not have purchased an extended warranty??? Sounds like you are a "BMW dealer for service" kid of person- in which case the warranty would have been money well spent.

Have you ever had a complaint about the seats? Any work every done to that area?

The dealer COULD have told BMW that it happened under warranty... do you have a key that wasn't used after the car hit 50k? Dealer could have logged the car in under that key (when they read it. the key will log the last miles when it was removed from the car.

What did the paperwork say on the documents your wife signed when she dropped it off...anything that would indicate it was represented as a warranty claim?

Find a new BMW mechanic. Not a dealer/.