In what circumstances will a court give an instruction that a mother was contributively negligent in a motor vehicle accident?

In Miller v. Peters, 37 Cal.2d 89, 230 P.2d 803, a mother and son sought damages for injuries resulting from a collision of automobiles. Defendant therein pleaded contributory negligence as to the son (the driver) but did not plead such negligence as to the mother (the passenger). In that case, the court gave an instruction with reference to contributory negligence on the part of the mother. Judgment was not defendant. The evidence therein was sufficient to prove that both plaintiffs (mother and son) were contributively negligent. In reversing the judgment, it was held that the introduction of evidence by defendant to the effect that both plaintiffs were contributively negligent did not authorize the giving of an instruction indicating that the mother could also be contributively negligent.