Honestly, I avoid Milkbone treats, but I don't think they would cause loose stools.
As for your cousin, if he thinks that giving your pup something not good for her is funny, advise him that he can pay the vet bills if he continues and she gets sick. If it were me I would make him sign an agreement...no lie! I don't take kindly to people who think it is funny to give things to pups that might make them ill.
