I have done that several times already: gone to the manager of the store
who can call out the license plate of the car and ask them to attend to their
animals, or if this doesn't get results, call the police.
In California it is against the law to leave animals in the car without adequate
ventilation, or in conditions which adversely affect the animal's health or welfare, so the police will fine the pet owner a hefty fee - rightfully so!