Although the transmission cooler lines most likely were ready for replacement at the time of transmission installation the operator should have noticed the oil leak in fairly short order unless it was an immediate catastrophic failure? The transmission oil pump pumps oil through the cooler under pressure so it is usually a spraying type leak. It is very noticeable especially when they are leaking enough to drain an entire transmission. Usually you can smell it and see it on the fenders long before it’s too late.
Its probably mechanic with lack of experience and general knowledge , failed transmission replaced without look around or thought about reason of failure ...happens all day long everywhere ...i hate when my truck broke down on the road ...most of the time get ready for rip off with unknown shop touching your truck . I replaced differential month ago on my peterbilt in wichita kansas and the guy was surprisingly affordable and fast ( i ordered weller to bring replacement next day he inspected the truck ) , so far all good except driver noticed drain (or filler) plug bit leaking . I got away at $3100 total
This is a litigious industry, they need a lawyer just as truck owners to deal with crap we all face. This is an issue where he needs legal advice on how to pursue recovery of damages, it actually reflects on all of us if your dad just accepts this as being ok, the dealer or shop who refuses to honor the warranty will pull this crap on someone else, and keep doing it until they are stopped. Screw them, having hoses replaced should be the norm, not the exception and their workmanship lacks enough that the legal avenue is the only way to fix the problem.
All I can recommend is to contact a lawyer that specializes in mechanical cases like this. Burden of proof is on the shoulders of the accuser, so your dad would have to prove negligence on behalf of the original repair. So the only thing on your side would be if the line simply loosened off or disconnected due to poor workmanship. If the line failed in any other way that could be due to age, then I think he is out of luck. I am suspecting this is the transmission cooler lines you are talking about. If the lines were unusable and have to be replaced, then yes, the original mechanic should have at least suggested it. But, if the line failed due to a defect he didn't see, then it would be hard to blame him. In my experience, most lines become seized and need to be cut off anyway. But I will be honest, most customers do not want new lines and think we are trying to scam them into paying for parts they think they don't need. So it is easy to see how new lines are not installed even when maybe they should be. However it works out, I wish your dad the best. Good luck!
I agree with what @Heavyd has said. The only other instance I can think off where the mechanic could be culpable is if the lines were installed in properly and they rubbed or chafed causing a catastrophic leak. For our own protection when we write work orders in our shop we always write a section of the mechanics recommendations with a full explanation of why. We then have the customer sign the work order to not only authorizing the repair but also acknowledging the mechanics recommendations.
The lawyer would cost more than two rebuilds of the gearbox. Aside from it being your duty, great job in helping your dad out.
When my clutch needed replacing, I took the transmission to a drivetrain shop, I had them go thru it(thinking preventative maintenance, it’s out anyways). Anyways, when I pick it up, the first thing the mechanic said was to replace the transmission cooler lines or else there is NO warranty.