
I'm really feeling for you - it's not good to ever lose a much loved pet, but especially when it's only young.
However, putting my (retired)breeder hat on, after 4 years, UNLESS you are able to confirm an hereditary problem there, I'd hate to have a finger pointed at me if a disaster like this happened. Of course, I'd store it in my 'to know' mind so if it started to happen to a sig.number of puppies I'd bred, I'd need to take steps to go in another direction with my breeding programme. What has happened is quite simply too long after the puppy was sold otherwise (assuming it was the normal age for a puppy to go to a new home?). Kidney failure quite often isn't seen until it's pretty advanced in any case. Which suggests that it was sudden, when it was not.
As said, there are many things that might cause this so you might think back re food that could cause this - like chocolate that, over time, can build up to the point organs are affected.
Puppies running around outside - well as soon as the weather allowed, we had our puppies out in their big ex-pen to run around which gave me a chance for a thorough clean-up of the whelping box, which with a big litter, was certainly needed. So quite frankly, there's nothing abnormal about a breeder doing this. I do question why your vet would suggest there was anything that happened then, would take 4 years to manifest itself!
Trying to find out the whys, when a dog suddenly collapses with something like kidney failure is understandable, but unfortunately with any living being, stuff just happens. All you can do is, if you go for another puppy, make sure you know all the things a breed might suffer with, and which can be tested for - and that the breeder DOES those tests before going ahead with a mating.
So sorry for you - it hurts.