I would contact your vet, if you have a specialist in this area even better and talk to them about the situation. I am not familiar with the condition but did google it and it says that it could be a combination of things similar to what causes HD. http://www.peteducation.com/article.cfm?c=2+1569&aid=464 I would probably get copies of the xrays sent to your vet for conformation and then try and work something out with the owner as there is no way to prove one way or the other what caused this problem. Both parents being healthy is not an issue, you could have both parents free of HD and a puppy crop up with it, sometimes crap happen but it is how you deal with it that will be of the most importance in the future. Learn as much as you can about the condition and talk to your owners with compassion as this is as hard for them as it is for you, it is their baby who is going to have to go through this and they are going to have to deal with keeping a lively puppy calm for the recovery period. If you had a contract I don't know what it says about genetic issues but this is one of those gray areas that you will have to take situation. Hope you are able to come to a resolution that you are both happy about.