If it was due to mats as they usually say..... I really think the groomers should instead of shaving them not do anything, then inform the owner when they return to see if the owner would perfer to comb out the knots themselves or have the dog shaved.
I have always been able to get the knots and tangles out with a little work. I would freak out if someone ever shaved my dogs
and it would be very very ugly.
One of my worst recurring nightmares is having one of my dogs shaved or a topknot cut off, lol That's how serious and upset I would be, so I groom myself.
Big hugs to the OP.