Basically, what this groomer did was brush your dog's teeth with an electric toothbrush which is perfectly legal. It is not unusual for groomers to use a Sonicare or other brand of electric tooth brush to clean a dog or cat's teeth. In fact, Philips, the manufacturer of this product recommends that it can be used to maintain dental hygiene, which is extremely important in maintaining the overall health of your dog (and cat if you have one)
In the state of California, groomers often provide this service and it is perfectly legal. Additionally, groomers will also provide other services that are often peformed in a vet's office, such as expression of a dog's anal glands and nail clipping.
The only legal question I have in regard to this matter is if this groomer has legitimate business license and if they are paying taxes to the California State Franchise Tax Board and of course, the IRS.
Pets are considered property by the legislative system for the most part. Even though animal welfare laws continue to evolve, mirroring the humanization trend, as a piece of property, a dog is not much different than an appliance when it comes to damage and loss. In California as a whole, the court system is more attuned to animal welfare than other states. This is the case for San Diego County. Even though I am in the San Francisco Bay area, I work extremely closely with a nationally know rescue group in El Cajon. You can always contact Consumer Affairs to report legitimate complaints against this groomer (or any other business for that matter) or also contact the BBB.
Should you have questions related to the practice utilized by this groomer, or if your dog suffered in any way,(meaning you had to take him to a vet and now have vet bills as a result of an injury) please let me know and I will be glad to assist you in any way I can.