What, when it comes to a rabbit does a vet mean when he say a "heve"?

So unfortunate that people don't ask of their vets what their vet can answer :( You are paying for a service and require all the information that you desire. Since the rabbit died you should take him back to the veterinarian and ask him what that meant. Also have a necropy done to see what happened. This will help you and will help the veterinarian to become more educated about whatever the rabbit had. Unfortunately rabbits are not very hardy. They sub quickly to bacterial infections or hairball impactions. The best rabbit I had was only three and died of a hair ball :(