They should maybe take him to the vet; something may be going on with his health. One of the places where I used to ride, one of the instructors had a big goofy black lab who loved everybody—until he was running around the back deck, slipped and ran head first into a retaining wall. Immediate personality change towards non-family: growling, snapping, hackles up, snarling, the whole shebang. I don’t think he ever bit anyone, but he damn sure threatened for several months. I figure he concussed himself or something.