I too once saw a pet being handed a little roughly by a groomer at Petco. I made eye contact with the groomer and gave her a look of disgust...very similar to your situation.
I think you did the 100% right thing!!!
I'm very glad you told the dogs owner!!! The poor dog doesn't have a voice...sometimes they need us to do it for them! You were his hero today!
I know there are very good and kind groomers out there....but it is my fear of
What I saw that keeps me doing all the grooming myself.