Liability in a dog bite case can be based on the doctrine of agency. “Agency is the relation that results from the act of one person, called the principal, who authorizes another, called the agent, to conduct one or more transactions with one or more third persons and to exercise a degree of discretion in effecting the purpose of the principal.” (L. Byron Culver & Associates v. Jaoudi Industrial & Trading Corp. (1991) 1 Cal.App.4th 300, 304 [1 Cal.Rptr.2d 680].)
The fact that a girlfriend, for example, was walking the dog without a leash made her the agent of the boyfriend, who had been taking care of his mother's dog. In the legal context of principal and agent, he was her principal. As such he is liable for her tort pursuant to California Civil Code section 2238 (“Unless required by or under the authority of law to employ that particular agent, a principal is responsible to third persons for the negligence of his agent in the transaction of the business of the agency, including wrongful acts committed by such agent in and as a part of the transaction of such business, and for his willful omission to fulfill the obligations of the principal.”)
This is not simply a California doctrine. The law of agency goes back hundreds of years and is fairly uniform throughout the United States and other English-speaking countries.