Causes quick and total failure. Most likely, your transmission was on its way out regardless of who looked at it. How many miles? What car and year? Had you done service before? What symptoms did you have that led you to the shop?

I've had a dealer misdiagnose a clogged cat as an alternator and also had to assist with diagnosing failed electrical components. In my opinion, outside of routine maintenance, dealers generally lack the experience of a skilled Indy shop and aren't my first choice for major issues (unless it's under warranty, their mode of troubleshooting is "replace at customers expense"). There is a great chance you don't even need a transmission swap, but it's easier for the tech to swap a transmission for $5k and a few hours than a seal for $150 and 5 hours more...

Also, the dealer quoted price is potentially half of what they are stating. Don't swap at the dealer, if you have to swap, go to a transmission shop. They'll have a better warranty, price, and experience.

I'd give your brother the benefit of the doubt. Should have returned the car to the first shop and NOT the dealer and let them look at it.