
Assuming this dog has been checked out by a vet and is not suffering any problems that could cause it to bite. Then it is a case of a dog developing a problem that has made it unsuitable to be used at stud.
Would think that the owner should stop using it and make sure all the owners of puppies it has produced should be contacted to let them know that the dog has now developed a temprement fault and the pups in turn have a fault in their background and should not be used for breeding. Likewise any pups they have produced should not be used to bred.