Here's my opinion, i would never let her get away with it!
It's like you would go to the dentist with your child and he/she accidently snipped a piece of your kids tong off!
scary to think of but very not acceptable!
Even if some may say... it's just a dog... well it isn't! it's your baby and he has feelings and rights too!
Actually last week i took Guidoune and Roxy to the groomers to have their nails clipped and she accidently snipped the vein (it happens), she was dripping blood and i could see my Guidoune (after she yelped) was very quiet and upset, it just reminded me that the job they can do i will do with my dremel here at home! and thats what i'm going to do!
I would do like many others sugested get legal advice!
And keep those pics!
Poor baby... yes acidents do happen... and nobody is perfect but they dont have to treat you like sh** after they do something horribly like that (not the groomer, i'm talking about the owner).
Very unproffesional.
Best of luck and big smoochies and hugs for your baby!