That's actually typical that they barely make it to the vet and then expire. The closest vet to me won't treat bunnies since she says they almost never make it and it gives her a really bad reputation even though there was nothing she could do since the bunnies are so far gone by the time they get to her.
All this happened in several hours? They have a fast metabolism (part of what makes it such a problem when they become sick) but I'd guess the bunny may have been hiding some symptoms for awhile before all this turned into an avalanche. As a prey species, they don't ever want to look sick or hurt since predators like to take out the weak ones.
We've never experienced all those symptoms together before but we don't feed sugar treats so we've not had diabetes to deal with so I couldn't guess if it was diabetes or not. The vet would have a much more educated opinion than me. Did they do tests?
Do you know what your customer was feeding the bunny? Maybe too many sweet treats? How long had the customer had the bunny? Several months or longer, we hope? How long does it take to create diabetes in bunnies?
I'm not a vet so my opinions mean nothing, but if we had a bunny with a runny eye, crusted nose and shallow breathing, it would sound like a cold and we'd feed it oatmeal.
The "can't walk, losing his bowels" along with the no cleaning sounds almost like a broken back? Won't eat or drink is a really sick rabbit or a rabbit wanting to die, the seizures could be from the whole pile of things wrong and may have been a symptom instead of a condition, but I'm not a vet and didn't meet the bunny. There may not be just one thing wrong, it could have been several?
Rabbits are a common laboratory animal, you'd expect some labs would want diabetic ones to test medicines on. Hmm, here's an article that had diabetes in NZ rabbits, but it was from severe inbreeding to create it:
https://www.ncbi.nlm.nih.gov/pubmed/7007801 Here's a medically created diabetes in rabbits,
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3014707/ however, again, the symptoms mentioned by your customer weren't mentioned.
Rabbits as a laboratory animal have created a huge amount of information about rabbit medicines and such, however it takes a bit to wade through the scientific language.
Hmm, a less medical terminology site, but here's their 'clinical signs for diabetes': (from:
http://www.medirabbit.com/EN/GI_diseases/Metabolic_diseases/Diabetes_en.htm )
"Clinical signs
In cases of experimentally induced diabetes, hyperglycemia is accompanied by polydipsia (excessive thirst), polyuria (excessive urination) and polyphagia (strong desire to eat)."
Well, just as a non-medical person, I'd guess it's not actually diabetes that killed the rabbit, but the vet probably has other things to do than to come up with an exact diagnoses for a dead patient. Especially one they saw for less than five minutes and did they charge for the visit? If they didn't, then they have even less reason to do an autopsy.
I'd guess your herd is safe from a genetic propensity for diabetes. If you sell another rabbit to that customer, perhaps a bit of rabbit care education may be in order? Especially in how to pick up and handle a rabbit, if it were me, I'd suspect a broken back was part of the issues. But, it's just me as a non-medical person on the other side of a computer screen so my opinion is of no real validity.